Planktic foraminiferal sedimentation and the marine calcite budget
Abstract
[1] The vertical flux and sedimentation rate of planktic foraminiferal tests are quantified and a global planktic foraminiferal CaCO3 budget is presented. Test and calcite flux rates are calculated according to the distribution of species obtained from multinet and sediment trap samples. Modern planktic foraminiferal population dynamics are discussed as a prerequisite for the quantification of the calcite budget, highlighting the importance of ecological, autecological (e.g., reproduction), and biogeochemical conditions that determine the presence or absence of species. To complete the open-marine, particulate CaCO3 inventory, the contribution of coccolithophores, pteropods, and calcareous dinophytes is discussed. Based on the studied regions, the global planktic foraminiferal calcite flux rate at 100 m depth amounts to 1.3–3.2 Gt yr−1, equivalent to 23–56% of the total open marine CaCO3 flux. The preservation of tests varies on a regional and temporal scale, and is affected by local hydrography and dissolution. During most of the year (off-peak periods), many tests dissolve above 700-m water depth while settling through the water column, with on average only 1–3% of the initially exported CaCO3 reaching the deep-seafloor. Pulsed flux events, mass dumps of fast settling particles, yield a major contribution of tests to the formation of deep-sea sediments. On average, ∼25% of the initially produced planktic foraminiferal test CaCO3 settles on the seafloor. The total planktic foraminiferal contribution of CaCO3 to global surface sediments amounts to 0.36–0.88 Gt yr−1, ∼32–80% of the total deep-marine calcite budget.
1. Introduction
[2] Planktic foraminifers (protists) and coccolithophores (gold-brown algae) are major contributors to the particulate inorganic calcite flux of the deep ocean, varying on regional and temporal scales [Deuser et al., 1981; Ziveri et al., 1995, 2000; Broerse et al., 2000]. Calcification and dissolution of CaCO3 produces changes in the surface water carbonate system, and deep-water masses are affected by the descent and differential dissolution of tests [e.g., Berger and Piper, 1972; Berger et al., 1982; Dittert et al., 1999; Dittert and Henrich, 2000]. Thermodynamic dissolution is evident below the lysocline, and below the calcite compensation depth (CCD) only a minor proportion of calcite is preserved [Broecker and Peng, 1982]. In addition, a significant amount of calcite dissolution takes place some distance above the calcite lysocline [Anderson and Sarmiento, 1994; Lohmann, 1995; Schiebel et al., 1997a; Milliman et al., 1999]. This may be caused by the remineralization of organic matter and decreasing pH within microenvironments. The decrease in test flux rate far above the planktic foraminiferal lysocline has hitherto not been sufficiently explained. On the other hand, calcareous particles settle below the CCD because they sink faster than they can be dissolved, and CaCO3 is well preserved in the surface sediments [cf. Archer, 1996]. Although the faunal record is biased, planktic foraminifers possess a high fossilization potential and have high biologic, ecologic, paleontologic, and stratigraphic significance [e.g., Bé, 1977; Vincent and Berger, 1981; Hemleben et al., 1989].
[3] Planktic foraminiferal test production and flux have an impact on and are affected by the turnover of oceanic and atmospheric CO2 [Hay, 1985; Veizer, 1985; Wolf-Gladrow et al., 1999a, 1999b]. When planktic foraminifers produce calcite from bicarbonate (HCO3−) or carbonate (CO32−), CO2 is released to the ambient water [Zeebe and Wolf-Gladrow, 2001]. As CO2 is the second most important greenhouse gas after water vapor, the production of planktic foraminifers and coccolithophores may affect the climate on seasonal to geological timescales [cf. Bramlette, 1958; Hay, 1985; Peterson and Prell, 1985; Henrich, 1986; Siegenthaler and Sarmiento, 1993; Denman et al., 1996; Archer et al., 2000; Schiebel and Hemleben, 2000]. This article discusses the population dynamics, production, and sedimentation of planktic foraminiferal tests and CaCO3, and gives a first-order estimate of the planktic foraminiferal contribution to the modern global marine carbonate budget.
2. Materials and Methods
[4] A total of 1777 multinet samples (>100 μm mesh size) and 27 sediment trap samples (>20–>100 μm) from the North Atlantic Ocean, the Caribbean, the Red Sea, and the Arabian Sea are included in the present study (Figure 1 and Tables 1–3). The sampling methods used for obtaining planktic foraminifers and pteropods by multinet and sediment trap, as well as the processing methods, are described in detail by Schiebel et al. [1995]. Standardized water depth intervals hauled with the multinet were 0–100, 0–700, and 0–2500 m, with five depth intervals each (see Table 4), enabling a direct comparison of data. The longest and best-resolved time series were obtained with a multinet device from the Eastern North Atlantic (BIOTRANS station, 47°N, 20°W) between 1988 and 1996, January through October (Table 1). The data from the Red Sea [Auras-Schudnagies et al., 1989; Bijma and Hemleben, 1994] and from the northern North Atlantic, including the Greenland-Norwegian Sea and the Arctic Ocean [Jensen, 1998; Volkmann, 2000], are included in the data set of planktic foraminiferal CaCO3 flux presented in this study. Daily to interannual data sets were combined, and regional data sets from different oceans were calibrated for their taxonomy and processing mode.
| Cruise | Year | Month | Location | No. of Samples | |
|---|---|---|---|---|---|
| Meteor 21-4 | 1992 | June | NE Atlantic | 62–75°N/14°W–20°E | 9 |
| Meteor 17-2 | 1991 | August | NE Atlantic | 50–74°N/20°W–28°E | 35 |
| Meteor 10-3 | 1989 | June–July | NE Atlantic | 72°N/10°W–7°E | 217 |
| Meteor 21-5 | 1992 | July | NE Atlantic | 67°N/3°E–17°W | 63 |
| Meteor 21-3 | 1992 | May | NE Atlantic | 57°N/20°W | 9 |
| Meteor 10-2 | 1989 | May–June | NE Atlantic | 57°N/22°W | 22 |
| Meteor 11-1 | 1989 | October | NE Atlantic | 48–55°N/16–34°W | 18 |
| Meteor 27-2 | 1994 | January | BIOTRANS | ∼47°N/20°W | 18 |
| Meteor 21-1 | 1992 | March–April | BIOTRANS | -′′- | 64 |
| Meteor 6-7 | 1988 | March–May | BIOTRANS | -′′- | 45 |
| Meteor 21-2 | 1992 | April–May | BIOTRANS | -′′- | 89 |
| Meteor 21-3 | 1992 | May | BIOTRANS | -′′- | 18 |
| Meteor 10-2 | 1989 | May–June | BIOTRANS | -′′- | 70 |
| Meteor 12-3 | 1990 | May–June | BIOTRANS | -′′- | 38 |
| Poseidon 200/6 | 1993 | June | BIOTRANS | -′′- | 5 |
| Meteor 10-4 | 1989 | July August | BIOTRANS | -′′- | 140 |
| Meteor 21-6 | 1992 | July August | BIOTRANS | -′′- | 52 |
| Meteor 26-1 | 1993 | September | BIOTRANS | -′′- | 12 |
| Meteor 36-5 | 1996 | September | BIOTRANS | -′′- | 176 |
| Meteor 36-6 | 1996 | October | BIOTRANS | -′′- | 26 |
| Poseidon 247/2 | 1999 | January | Azores Front | 32–35°N/20–32°W | 91 |
| Poseidon 231/3 | 1997 | August | Azores Front | 32.5–36°N/22–34.5°W | 44 |
| ArqFCA97C | 1997 | August | Azores Front | 32–35°N/29–32°W | 20 |
| Meteor 42-3 | 1998 | August | Azores Front | 32–35°N/29–32°W | 51 |
| Meteor 10-1 | 1989 | March–April | Subtropical Atlantic | 33°N/20°W | 8 |
| Meteor 21-3 | 1992 | May | Subtropical Atlantic | 33°N/20°W | 13 |
| Meteor 36-2 | 1996 | May–June | Subtropical Atlantic | 33°N/22°W | 5 |
| Meteor 10-1 | 1989 | March–April | Subtropical Atlantic | 18°N/30°W | 5 |
| Meteor 35-1 | 1996 | April–May | Caribbean | 12–19°N/61–79°N | 129 |
| Total | 1492 | ||||
| Cruise | Year | Month | Location | No. of Samples |
|---|---|---|---|---|
| Meteor 31-2 | 1995 | February | Red Sea | 50 |
| Meteor 31-3 | -′′- | March | Gulf of Aden, off Oman | 75 |
| Meteor 32-3 | -′′- | May | off Oman, 0°–21°N, 65°E, Seychelles | 13 |
| Meteor 32-5 | -′′- | July August | off Oman, WAST, CAST, SAST | 18 |
| Meteor 33-1 | -′′- | September–October | NAST, WAST, CAST, EAST, SAST | 52 |
| Sonne 117 | 1997 | March | 65°E, SAST, CAST, WAST | 26 |
| Sonne 118 | -′′- | April–May | NAST, WAST, CAST, SAST | 26 |
| Sonne 119 | -′′- | May–June | NAST, WAST, CAST, EAST, SAST | 25 |
| Total | 285 |
- a Stations are located at 20°N, 65°40′E (NAST), 16°32′N, 55°19′E (WAST), 14°25′N, 64°34′E (CAST), 15°40′N, 68°35′E (EAST), and 10°N, 65°E (SAST).
| Mooring | Trap No. | Water Depth, m | Year | Cup | Bulk Weight | Split Weight | Time Interval | CD, μm | plf Tests, × 103 day−1 | plf CaCO3, mg m−2 d−1 |
|---|---|---|---|---|---|---|---|---|---|---|
| L2-92-A | 42 | 1000 | 1992 | 3 | 783.0 | 6.4 | 4.4.–12.4. | >20 | 24.4 | 164.6 |
| 8 | 356.5 | 8.0 | 17.5.–22.5. | >20 | 14.6 | 65.7 | ||||
| 12 | 497.7 | 7.1 | 15.6.–23.6. | >20 | 14.3 | 91.9 | ||||
| L2-92-A | 43 | 3500 | 1992 | 7 | 375.1 | 5.2 | 6.5.–14.5. | >20 | 9.7 | 60.2 |
| 10 | 306.1 | 6.7 | 30.5.–7.6. | >20 | 5.1 | 19.7 | ||||
| 14 | 444.2 | 13.1 | 1.7.–9.7. | >20 | 10.9 | 62.5 | ||||
| L2-92-B | 46 | 3530 | 1992 | 2 | 1559 | 13.7 | 10.6.–8.7. | >20 | 13.1 | 67.5 |
| 3 | 1536 | 7.5 | 8.7.–5.8. | >20 | 19.8 | 106.9 | ||||
| 4 | 748.1 | 8.0 | 5.8.–2.9. | >20 | 12.2 | 65.0 | ||||
| 5 | 926.1 | 6.4 | 2.9.–30.9. | >20 | 11.8 | 60.9 | ||||
| 6 | 802 | 5.5 | 30.9.–28.10. | >20 | 11.4 | 51.5 | ||||
| 7 | 407.3 | 4.6 | 28.10.–25.11. | >20 | 10.5 | 19.6 | ||||
| 8 | 404.1 | 7.8 | 25.11.–23.12. | >30 | 22.3 | 25.3 | ||||
| 1993 | 13 | 484.4 | 7.3 | 31.3.–14.4. | >30 | 16.5 | 9.4 | |||
| 16 | 401.8 | 9.3 | 12.5.–26.5. | >64 | 1.0 | 10.7 | ||||
| L2-92-B | 49 | 1030 | 1992 | 2 | 1161 | 6.4 | 10.6.–8.7. | >20 | 16.5 | 47.8 |
| 3 | 931.2 | 10.8 | 8.7.–5.8. | >20 | 11.1 | 29.8 | ||||
| 4 | 690.0 | 9.3 | 5.8.–2.9. | >20 | 11.5 | 42.4 | ||||
| L2-92-B | 50 | 2030 | 1992 | 2 | 1997 | 8.8 | 10.6.–8.7. | >20 | 43.7 | 99.3 |
| 3 | 1445 | 12.9 | 8.7.–5.8. | >20 | 33.0 | 81.9 | ||||
| 4 | 851.1 | 9.0 | 5.8.–2.9. | >20 | 14.1 | 68.2 | ||||
| 6 | 419.7 | 6.9 | 30.9.–28.10. | >20 | 9.8 | 37.6 | ||||
| 1993 | 9 | 682.2 | 8.1 | 23.12.–20.1. | >20 | 7.2 | 6.7 | |||
| 16 | 1587 | 20.8 | 12.5.–26.5. | >20 | 14.0 | 29.6 | ||||
| L2-96 | 62 | 2000 | 1996 | 4 | 912.0 | 5.1 | 1.9.–14.9. | >100 | 4.3 | 7.5 |
| 5 | 964.6 | 4.2 | 15.9.–28.9. | >100 | 4.7 | 13.0 | ||||
| L3-92 | 45 | 2200 | 1992 | 2 | 1967 | 23.5 | 10.6.–8.7. | >20 | 81.1 | 81.1 |
- a Samples were recovered by the marine chemistry working group at IfM Kiel from the North Atlantic at 47°50′N, 19°39′W (L2) and 54°N, 21°W (L3). Weight of samples (bulk and split) is brutto dry weight (mg), including sea-salt. Census data (CD) are collected from different size fractions (>20 μm) according to the presence of tests. CaCO3 weight of planktic foraminifers (plf) is calculated according to test weight given by Schiebel and Hemleben [2000].
| Latitude | Longitude | Cruise | Month | 20 | 40 | 50 | 60 | 80 | 100 | 150 | 200 | 300 | 350 | 400 | 500 | 550 | 600 | 700 | 750 | 800 | 1000 | 1100 | 1500 | 2000 | 2500 | 3000 | 4000 | 4282 | 4500 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Atlantic | |||||||||||||||||||||||||||||
| 75°–77°N | 5°–10°E | MET21/4 | June | 20.0 | 29.6 | … | 24.8 | 10.2 | 9.0 | … | 5.8 | 2.6 | … | … | 1.6 | … | … | 3.4 | … | … | 0.5 | … | 1.5 | 2.2 | … | … | … | … | … |
| 70°–75°N | 11°W | MET21/4 | June | 3.5 | 27.5 | … | 16.6 | 10.1 | 16.3 | … | 4.1 | 1.9 | … | … | 1.1 | … | … | 0.7 | … | … | … | … | … | … | … | … | … | … | … |
| 70°–75°N | 15°–20°E | MET21/4 | June | 10.6 | 8.6 | … | 9.3 | 26.9 | 13.9 | … | 2.0 | 1.9 | … | … | 1.8 | … | … | 0.8 | … | … | … | … | … | … | … | … | … | … | … |
| 70°–72°N | 0°–5°E | MET10/3 | June | 91.6 | 15.1 | … | 116.3 | 80.1 | 20.1 | … | 4.2 | 2.3 | … | … | 1.2 | … | 6.3 | … | … | … | … | 7.2 | … | … | 7.4 | … | … | … | … |
| 70°–72°N | 0°–5°W | MET10/3 | June | 328.2 | 424.5 | … | 93.9 | 63.6 | 104.5 | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 70°–72°N | 5°–10°E | MET10/3 | June | 162.1 | 198.3 | … | 288.6 | 171.1 | 230.3 | … | 248.3 | 65.1 | … | … | 19.2 | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 70°–72°N | 5°–10°W | MET10/3 | June | 93.5 | 98.0 | 63.0 | 190.4 | 188.8 | 163.5 | … | 82.2 | 30.2 | … | … | 11.8 | … | … | … | … | … | 4.8 | … | 3.5 | 4.7 | 6.0 | … | … | … | … |
| 70°–72°N | 5°–10°W | MET10/3 | July | 13.7 | 23.3 | … | 85.5 | 116.8 | 150.4 | … | 90.0 | 33.6 | … | … | 6.7 | … | … | … | 3.0 | … | 4.8 | … | 5.0 | 0.5 | 3.6 | … | … | … | … |
| 70°–72°N | 5°–10°W | MET21/5 | July | 14.6 | 18.2 | … | 16.6 | 4.0 | 15.1 | … | 16.8 | 5.6 | … | … | 2.0 | … | … | 1.0 | … | … | 1.6 | … | 0.6 | 1.0 | 0.8 | … | … | … | … |
| 70°N | 0°W | MET21/4 | June | 34.0 | … | … | 85.9 | 61.0 | 29.7 | … | 4.8 | 3.0 | … | … | 2.5 | … | … | 1.0 | … | … | … | … | … | … | … | … | … | … | … |
| 70°N | 0°W | MET21/5 | July | 22.9 | 19.5 | … | 14.0 | 7.5 | 9.1 | … | 2.4 | 2.2 | … | … | 1.5 | … | … | 1.2 | … | … | 6.2 | … | 4.9 | 2.9 | 1.9 | … | … | … | … |
| 69°N | 0°W | MET21/5 | July | 84.5 | 133.1 | … | 134.1 | 114.0 | 57.4 | … | 34.1 | 4.8 | … | … | 5.9 | … | … | 10.9 | … | … | … | … | … | … | … | … | … | … | … |
| 67°N | 3°E | MET21/5 | July | 34.9 | 47.8 | … | 14.2 | 9.6 | 250.2 | … | 12.5 | 14.0 | … | … | 2.2 | … | … | 4.2 | … | … | … | … | … | … | … | … | … | … | … |
| 67°N | 5°E | MET21/4 | June | 345.3 | 121.4 | … | 433.6 | 16.4 | 27.6 | … | 14.2 | 12.3 | … | … | 4.8 | … | … | 5.8 | … | … | … | … | … | … | … | … | … | … | … |
| 65°–70°N | 4°W | MET21/4 | June | 211.7 | 137.1 | … | 179.8 | 21.4 | 5.0 | … | 2.3 | 6.6 | … | … | 1.6 | … | … | 1.0 | … | … | … | … | … | … | … | … | … | … | … |
| 62°–65°N | 9°–14°W | MET21/4 | June | 73.7 | 16.2 | … | 13.6 | 28.3 | 39.2 | … | 5.7 | 0.8 | … | … | 0.8 | … | … | 1.6 | … | … | … | … | … | … | … | … | … | … | … |
| 57°N | 20°–22°W | MET10/2 | May | 40.7 | 93.6 | 34.2 | 123.9 | 162.1 | 127.8 | … | 214.4 | 136.0 | … | … | 71.0 | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 57°N | 20°–22°W | MET21/3 | May | 416.2 | 565.3 | … | 548.1 | 580.4 | 525.9 | … | 313.4 | 198.7 | … | … | 111.8 | … | … | 129.9 | … | … | … | … | … | … | … | … | … | … | … |
| 57°N | 20°–22°W | MET10/2 | June | … | … | … | … | … | … | … | … | … | … | … | 13.8 | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 55°N | 27°–32°W | MET11/1 | Oct. | … | … | … | … | … | … | … | 38.7 | 14.7 | … | … | 11.0 | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 54°N | 16°–18°W | MET11/1 | Oct. | … | … | … | … | … | … | … | 132.0 | 23.3 | … | … | 37.9 | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 51°N | 34°W | MET11/1 | Oct. | … | … | … | … | … | … | … | 8.4 | 2.9 | … | … | 6.3 | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 48°N | 27°W | MET11/1 | Oct. | … | … | … | … | … | … | … | 3.0 | 0.8 | … | … | 2.4 | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 47°N | 20°W | MET27/2 | Jan. | 5.4 | 0.0 | … | 6.0 | 3.5 | 3.7 | … | 6.1 | 1.8 | … | … | 1.8 | … | … | 1.0 | … | … | 0.3 | … | 0.2 | … | 0.4 | … | … | … | … |
| 47°N | 20°W | MET21/1–2 | March | 68.6 | 37.1 | … | 84.2 | 53.8 | 43.7 | … | 41.0 | 34.1 | … | … | 4.3 | … | … | 3.1 | … | … | 1.9 | … | 1.3 | 0.8 | 0.2 | … | … | 6.1 | … |
| 47°N | 20°W | MET21/1–2 | April | 50.0 | 35.0 | … | 40.0 | 42.0 | 50.4 | … | 29.9 | 18.1 | … | … | 6.1 | … | … | 6.0 | … | … | 2.2 | … | 3.5 | 2.4 | 1.9 | … | … | … | 0.004 |
| 47°N | 20°W | MET6/7 | April | … | … | 166.7 | … | … | 130.0 | 131.9 | 157.7 | 87.5 | 61.2 | … | 53.9 | 74.7 | … | … | … | 48.8 | … | … | 24.6 | … | … | 11.0 | … | … | … |
| 47°N | 20°W | MET6/7 | May | … | … | 11.7 | … | … | 20.1 | 15.1 | … | … | 3.2 | … | 9.2 | … | … | … | … | … | 38.6 | … | 10.6 | 7.7 | … | … | … | … | … |
| 47°N | 20°W | MET10/2 | May | 123.5 | 155.8 | … | 133.3 | 89.8 | 89.7 | … | 56.4 | 35.5 | … | 22.0 | 21.9 | … | … | … | … | … | 20.1 | … | 21.9 | 9.7 | … | 2.4 | … | … | … |
| 47°N | 20°W | MET12/3 | May | … | … | … | … | … | … | … | 10.9 | 10.8 | … | … | 3.1 | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 47°N | 20°W | MET21/1–2 | May | 23.0 | 8.6 | … | 14.4 | 21.0 | 31.5 | … | 22.4 | 12.0 | … | … | 6.4 | … | … | 5.2 | … | … | 4.1 | … | 2.6 | 3.6 | 2.1 | … | … | … | … |
| 47°N | 20°W | MET21/3 | May | 109.9 | 99.9 | … | 179.5 | 173.7 | 90.8 | … | 29.3 | 16.4 | … | … | 7.1 | … | … | 8.5 | … | … | 2.9 | … | 3.9 | 3.4 | 3.1 | … | … | … | … |
| 47°N | 20°W | MET12/3 | June | 386.0 | 112.1 | 161.0 | 57.6 | 78.4 | 35.1 | … | 8.7 | 15.4 | … | … | 1.0 | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 47°N | 20°W | POS200/6 | June | 52.7 | 29.0 | … | 39.7 | 5.9 | 25.8 | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 47°N | 20°W | MET10/4 | Aug. | 12.8 | 13.6 | … | 10.3 | 4.9 | 4.2 | … | 0.4 | 0.2 | … | … | 0.1 | … | … | … | … | … | 0.026 | … | 0.015 | 0.010 | … | 0.003 | 0.003 | … | … |
| 47°N | 20°W | MET21/6 | Aug. | 113.2 | 349.4 | … | 47.7 | 37.8 | 46.0 | … | 17.8 | 15.2 | … | … | 7.1 | … | … | 4.4 | … | … | 5.8 | … | 5.3 | 8.9 | 9.2 | … | … | … | … |
| 47°N | 20°W | MET26/1 | Sept. | 106.5 | 71.3 | … | 82.2 | 86.1 | 111.4 | … | 44.4 | 11.7 | … | … | 5.9 | … | … | 5.0 | … | … | 11.6 | … | 9.5 | 3.1 | … | … | … | … | … |
| 47°N | 20°W | MET36/5 | Sept. | 104.9 | 100.8 | … | 101.6 | 87.0 | 24.2 | … | 10.1 | 8.0 | … | … | 4.3 | … | … | 4.3 | … | … | 7.1 | … | 4.6 | 3.2 | 6.0 | … | … | … | … |
| 47°N | 20°W | MET36/5 | Oct. | 169.5 | 196.6 | … | 183.4 | 71.5 | 43.4 | … | 13.2 | 8.4 | … | … | 3.0 | … | … | 6.7 | … | … | 12.4 | … | 8.0 | … | 5.5 | … | … | … | … |
| 47°N | 20°W | MET36/6 | Oct. | 60.4 | 48.9 | … | 61.1 | 58.5 | 31.6 | … | 9.9 | 3.0 | … | … | 2.9 | … | … | 2.8 | … | … | 6.0 | … | 2.5 | 2.4 | 2.8 | … | … | … | … |
| 36°N | 20.5°W | POS247/2 | Jan. | 540.5 | 128.3 | … | 124.7 | 136.7 | 189.7 | … | 234.1 | 92.0 | … | … | 9.0 | … | … | 8.4 | … | … | 4.3 | … | 1.4 | 0.6 | 3.4 | … | … | … | … |
| 35°N | 30°W | POS247/2 | Jan. | 143.2 | 89.3 | … | 110.7 | 172.4 | 164.0 | … | 112.8 | 18.6 | … | … | 11.1 | … | … | 13.6 | … | … | 9.4 | … | 1.1 | 1.4 | 1.5 | … | … | … | … |
| 35°N | 30°W | MET 42/3 | Aug. | 56.4 | 40.8 | … | 33.8 | 23.3 | 35.4 | … | 7.1 | 4.2 | … | … | 4.7 | … | … | 2.9 | … | … | 3.5 | … | 2.2 | 2.3 | 1.0 | … | … | … | … |
| 35°N | 30°W | POS231/3 | Aug. | 7.9 | 9.5 | … | 9.3 | 27.5 | 41.8 | … | 2.1 | 2.0 | … | … | 1.9 | … | … | 1.2 | … | … | 2.6 | … | 0.8 | 0.9 | 0.9 | … | … | … | … |
| 35°N | 30°W | FCA97C | Aug. | 1.9 | 1.6 | … | 1.4 | 14.3 | 21.9 | … | 1.8 | 3.5 | … | … | 2.4 | … | … | 1.1 | … | … | … | … | … | … | … | … | … | … | … |
| 33°N | 22°W | POS247/2 | Jan. | 164.9 | 5.7 | … | 22.6 | 106.3 | 127.1 | … | 146.2 | 29.4 | … | … | 23.6 | … | … | 18.1 | … | … | 14.1 | … | 1.9 | 1.2 | 4.0 | … | … | … | … |
| 33°N | 22°W | MET21/3 | May | 21.0 | 16.9 | … | … | 54.4 | 51.5 | … | … | 25.5 | … | … | 5.5 | … | … | 6.0 | … | … | 5.4 | … | 1.8 | 1.7 | 1.7 | … | … | … | … |
| 33°N | 22°W | MET36/2 | May | 0.014 | 0.005 | … | 0.003 | 0.008 | 0.025 | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 33°N | 22°W | MET36/2 | June | 3.5 | 1.2 | … | 0.5 | 1.5 | 5.0 | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 33°N | 22°W | POS231/3 | Aug. | 3.6 | 5.5 | … | 9.7 | 20.1 | 12.8 | … | 0.8 | 0.7 | … | … | 1.0 | … | … | 0.5 | … | … | 0.5 | … | 0.5 | 0.4 | 0.6 | … | … | … | … |
| 33.5°N | 26°W | POS247/2 | Jan. | 164.9 | 5.7 | … | 22.6 | 106.3 | 127.1 | … | 146.2 | 29.4 | … | … | 23.6 | … | … | 18.1 | … | … | 450.5 | … | 1.9 | 1.2 | 4.0 | … | … | … | … |
| 32°N | 31°W | POS247/2 | Jan. | 257.6 | 30.8 | … | 58.0 | 97.7 | 187.6 | … | 89.1 | 39.7 | … | … | 11.3 | … | … | 5.9 | … | … | 8.9 | … | 1.0 | 1.0 | 1.7 | … | … | … | … |
| 32°N | 31°W | MET 42/3 | Aug. | 9.3 | 4.8 | … | 4.4 | 10.6 | 0.9 | … | 1.6 | 2.1 | … | … | 1.3 | … | … | 0.9 | … | … | 0.4 | … | 0.1 | 0.1 | 0.1 | … | … | … | … |
| 32°N | 31°W | FCA97C | Aug. | 2.5 | 0.5 | … | 1.4 | 6.3 | 3.6 | … | 0.1 | 0.5 | … | … | 0.2 | … | … | 0.6 | … | … | 0.3 | … | 0.5 | … | … | … | … | … | … |
| 32.5°N | 34.5°W | POS231/3 | Aug. | 4.6 | 4.8 | … | 13.1 | 9.8 | 3.1 | … | 2.4 | 2.3 | … | … | 3.0 | … | … | 3.9 | … | … | 2.0 | … | 0.9 | 1.5 | 0.3 | … | … | … | … |
| 18°N | 30°W | MET10/1 | April | 19.2 | 7.8 | … | 9.0 | 3.4 | 0.9 | … | 37.5 | 10.7 | … | … | 7.1 | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| Arabian Sea and Red Sea | |||||||||||||||||||||||||||||
| 27°N | 35°E | MET31/2 | Feb. | 48.6 | 10.3 | … | 81.0 | 108.7 | 30.7 | … | 51.0 | 8.7 | … | … | 3.7 | … | … | 5.3 | … | … | … | … | … | … | … | … | … | … | … |
| 20°N | 60°E | So119 | May | 266.9 | 64.0 | … | 61.9 | 37.1 | 65.3 | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 2°N | 65°E | Met32/3 | May | 239.3 | 363.2 | … | 357.6 | 234.9 | 98.8 | … | 85.8 | 68.6 | … | … | 23.5 | … | … | 14.9 | … | … | 20.1 | … | 38.2 | 27.6 | 26.1 | … | … | … | … |
| 17.5°N | 58.5°E | So119 | May | 410.9 | 470.8 | … | 375.9 | 235.6 | 125.1 | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 16°N | 60°E | Met31/3 | March | 254.2 | 181.2 | … | 134.2 | 45.3 | 39.1 | … | 31.7 | 14.4 | … | … | 10.2 | … | … | 10.0 | … | … | 8.7 | … | 1614.6 | 1592.9 | 1736.3 | … | … | … | … |
| 16°N | 60°E | So117 | March | 30.1 | 76.4 | … | 112.8 | 38.7 | 17.7 | … | 13.6 | 8.5 | … | … | 5.6 | … | … | 9.6 | … | … | 8.3 | … | 6.1 | 5.7 | 7.8 | … | … | … | … |
| 16°N | 60°E | So118 | April | 6.9 | 3.0 | … | 4.9 | 4.2 | 2.6 | … | 3.6 | 1.4 | … | … | 1.5 | … | … | 2.5 | … | … | 19.3 | … | 41.1 | 3.3 | 5.9 | … | … | … | … |
| 16°N | 60°E | So119 | May | 565.8 | 544.1 | … | 180.1 | 20.2 | 36.8 | … | 46.4 | 38.3 | … | … | 16.4 | … | … | 10.5 | … | … | 21.4 | … | 5.4 | 10.8 | 2.3 | … | … | … | … |
| 16°N | 60°E | Met33/1 | Sept. | 366.8 | 284.7 | … | 539.9 | 51.1 | 80.2 | … | 30.9 | 39.1 | … | … | 3.6 | … | … | 58.9 | … | … | 40.3 | … | 20.2 | 24.2 | 33.1 | … | … | … | … |
| 16°N | 69°E | Met33/1 | Oct. | 22.0 | 12.5 | … | 6.0 | 4.6 | 12.5 | … | 1.4 | 0.8 | … | … | 0.9 | … | … | 0.6 | … | … | 1.3 | … | 1.3 | 2.2 | 2.4 | … | … | … | … |
| 15°N | 52°–54°E | Met31/3 | March | 733.9 | 742.2 | … | 711.9 | 329.8 | 194.9 | … | 130.4 | 90.2 | … | … | 9.8 | … | … | 9.7 | … | … | 6.4 | … | 2.9 | 3.6 | 6.8 | … | … | … | … |
| 14°N | 65°E | So119 | May | 242.9 | 326.1 | … | 232.7 | 227.7 | 163.0 | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 14°N | 65°E | Met32/5 | Aug. | 343.1 | 333.3 | … | 317.1 | 325.0 | 180.9 | … | 15.5 | 6.6 | … | … | 12.3 | … | … | 13.1 | … | … | 13.0 | … | 10.0 | 9.5 | 9.0 | … | … | … | … |
| 14°N | 65°E | Met33/1 | Oct. | 84.5 | 92.1 | … | 26.5 | 5.1 | 5.3 | … | 4.2 | 3.6 | … | … | 1.5 | … | … | 3.5 | … | … | 3.6 | … | 1.5 | 4.8 | 4.6 | … | … | … | … |
| 13°N | 47°–50°E | Met31/3 | March | 184.1 | 198.9 | … | 207.4 | 155.2 | 98.8 | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 12°N | 44°E | Met31/3 | March | 122.0 | 160.6 | … | 123.3 | 51.2 | 29.7 | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 10°N | 65°E | So117 | March | 19.0 | 10.4 | … | 7.6 | 10.8 | 24.1 | … | 12.6 | 3.4 | … | … | 5.6 | … | … | 1.5 | … | … | 5.2 | … | 4.6 | 6.8 | 9.2 | … | … | … | … |
| 10°N | 65°E | So118 | April | 79.8 | 61.4 | … | 14.1 | 37.2 | 24.2 | … | 8.7 | 5.2 | … | … | 7.0 | … | … | 1.8 | … | … | 6.1 | … | 3.7 | 3.3 | 2.2 | … | … | … | … |
| 10°N | 65°E | So119 | May | 259.6 | 254.3 | … | 130.1 | 80.6 | 97.3 | … | 24.4 | 32.7 | … | … | 5.0 | … | … | 9.7 | … | … | 5.3 | … | 4.1 | 3.0 | 2.9 | … | … | … | … |
| 10°N | 65°E | Met33/1 | Oct. | 3.3 | 10.8 | … | 14.2 | 1.4 | 7.1 | … | 4.2 | 1.8 | … | … | 0.3 | … | … | 0.8 | … | … | 1.3 | … | 1.5 | 1.5 | 1.8 | … | … | … | … |
| Caribbean | |||||||||||||||||||||||||||||
| 19°N | 63.7°W | MET35/1 | April | 1.4 | 0.4 | … | 0.4 | 0.2 | 0.6 | … | 1.5 | 0.4 | … | … | 0.2 | … | … | 1.0 | … | … | 0.3 | … | 0.3 | 0.3 | 0.4 | … | … | … | … |
| 18°N | 63.6°W | MET35/1 | April | 89.5 | 98.3 | … | 50.9 | 41.5 | 16.3 | … | 3.3 | 2.8 | … | … | 2.0 | … | … | 4.1 | … | … | … | … | … | … | … | … | … | … | … |
| 18°N | 65°W | MET35/1 | April | 36.8 | 53.8 | … | 37.8 | 39.6 | 28.9 | … | 12.4 | 3.1 | … | … | 4.8 | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 18°N | 67.5°W | MET35/1 | May | 36.3 | 15.8 | … | 35.9 | 11.5 | 12.5 | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 18.8°N | 64°W | MET35/1 | April | 6.7 | 2.9 | … | 7.0 | 1.9 | 9.9 | … | 0.6 | 0.2 | … | … | 0.6 | … | … | 0.6 | … | … | … | … | … | … | … | … | … | … | … |
| 18.3°N | 63.6°W | MET35/1 | April | 8.4 | 1.3 | … | 0.4 | 1.8 | 1.9 | … | 1.0 | 0.5 | … | … | 0.6 | … | … | 1.2 | … | … | … | … | … | … | … | … | … | … | … |
| 17°N | 65.5°W | MET35/1 | May | 71.0 | 33.8 | … | 15.3 | 15.9 | 9.5 | … | 14.1 | 8.5 | … | … | 4.8 | … | … | 7.3 | … | … | 5.4 | … | 2.6 | 2.2 | 3.6 | … | … | … | … |
| 17.6°N | 65.5°W | MET35/1 | May | 262.2 | 137.7 | … | 125.5 | 72.6 | 72.2 | … | 11.8 | 5.7 | … | … | 5.1 | … | … | 0.9 | … | … | … | … | … | … | … | … | … | … | … |
| 17.6°N | 79.2°W | MET35/1 | May | 44.9 | 90.5 | … | 52.0 | 49.1 | 28.9 | … | 12.5 | 6.2 | … | … | 2.3 | … | … | 1.2 | … | … | … | … | … | … | … | … | … | … | … |
| 17.5°N | 64.2°W | MET35/1 | April | 22.4 | 13.4 | … | 5.5 | 4.8 | 12.9 | … | 0.9 | 0.3 | … | … | 0.4 | … | … | 1.3 | … | … | … | … | … | … | … | … | … | … | … |
| 16.5°N | 62.5°W | MET35/1 | April | 2.6 | 5.2 | … | 11.2 | 7.3 | 2.1 | … | 2.0 | 1.8 | … | … | 4.2 | … | … | 2.1 | … | … | … | … | … | … | … | … | … | … | … |
| 15.5°N | 62.2°W | MET35/1 | April | 24.6 | 17.2 | … | 14.4 | 22.0 | 1.5 | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … | … |
| 14.5°N | 61.5°W | MET35/1 | April | 0.5 | 7.3 | … | 2.4 | 0.3 | 10.5 | … | 5.6 | 5.8 | … | … | 3.0 | … | … | 3.5 | … | … | 3.8 | … | 3.9 | 4.9 | 6.3 | … | … | … | … |
| 12°N | 61.2°W | MET35/1 | April | 9.7 | 10.5 | … | 20.8 | 13.1 | 1.4 | … | 1.0 | 2.9 | … | … | 2.8 | … | … | 3.3 | … | … | … | … | … | … | … | … | … | … | … |
- a Expressed in mg m−2 d−1 and given as monthly averages for different water depths and locations in the North Atlantic, Arabian and Red Seas, and the Caribbean. Flux rates refer to a test size of >100 and >125 μm in the case of METEOR cruise 10-2 and SONNE cruise 119.
[5] Sediment trap samples (Table 3), deployed by the Marine Chemistry working group of the IfM Kiel [Lundgreen, 1996] were analyzed and flux rates of planktic foraminiferal tests and the resulting CaCO3 are compared with flux rates calculated from multinet samples. Both methods have inherent advantages and disadvantages in temporal and spatial sampling resolution. The sediment trap methodology, including trapping efficiency, has been thoroughly discussed [see Lundgreen, 1996, and references therein; Scholten et al., 2001]. Census data are available from the Pangaea database at the AWI (Bremerhaven, Germany, http://www.pangaea.de/home/rschiebel/). Planktic foraminiferal calcite flux rates from key regions of the global deep ocean have been included from the literature (Figure 1 and Table 5).
| Location | Water depth, m | plf CaCO3, Flux | % of total CaCO3, Flux | Test Size, μm | Loc. | Reference |
|---|---|---|---|---|---|---|
| 47°N, 20°W | 1000 | 1.83 | 17 | 150–250 | B | Honjo and Manganini [1993] |
| 2000 | 2.27 | 15 | ||||
| 3749 | 1.77 | 12 | ||||
| 33°N, 22°W | 1000 | 13 | 15.14 | 150–250 | A | Honjo and Manganini [1993] |
| 2000 | 13.8 | 4.49 | ||||
| 48°N, 21°W | 1018–3749 | 1.8 | 40 | >0.45 | Broerse et al. [2000] | |
| 34°N, 21°W | 1070–4564 | 4.2 | 45 | >0.45 | ||
| 50°N, 145°W | surface | 1.7 | 8 | 1 | Reynolds and Thunell [1985] | |
| (Station PAPA) | 3800 | 2.2 | 20 | |||
| 50°N, 145°W | 3800 | 2.2 | 18.8 | 63–1000 | 1 | Thunell and Honjo [1987] |
| 39.5°N, 128°W | 4050 | ∼1.4 | 38 | >150 | 2 | Fischer et al. [1983] |
| 33°33′N, 118°30′W | 500 | 22 | 75 | >125 | 3 | Ziveri et al. [1995] |
| 5°21′N, 81°53′W | 890 | 4.9 | 4.3 | 4 | Thunell and Reynolds [1984] | |
| 3560 | 1.9 | 11 | ||||
| 13°31′N, 54°0′W | 389 | 3.61 | 62.7 | >100 | 5 | Thunell and Honjo [1981] |
| 988 | 2.6 | 40.8 | ||||
| 3755 | 4.86 | 88.1 | ||||
| 5068 | 2.67 | 64.6 | ||||
| 15°21′N, 151°28′W | 978 | 0.73 | 62.5 | >100 | 5 | Thunell and Honjo [1981] |
| 2778 | 0.73 | 47.6 | ||||
| 4280 | 0.62 | 42.5 | ||||
| 5582 | 0.8 | 100 | ||||
| 30°1′S, 73°11′W | 2173 | 8.0 | 29.5 | 6 | Marchant [1995] | |
| 60°55′S, 57°6′W | sea surface | 5.5 | ∼100 | 7 | Wefer et al. [1982] | |
| 965 | 4.5 | |||||
| 3625 | 6.6 | |||||
| 62°27′S, 34°46′W | 3880 | < 0.01 | ? | 8 | Fischer et al. [1988] | |
| 2°47′N, 8°51′W | 388 | 0.22 | 2.0 | >53 | 9 | Bishop et al. [1977] |
| 21°9′N, 20°41′W | 730 | ∼5 | 33 | 10 | Fischer et al. [1996] | |
| 20°45′N, 19°45′W | 2195 | ∼8 | 33 | |||
| 21°9′N, 20°41′W | ∼3500 | ∼7 | 33 | |||
| 32°5′N, 64°15′W | 3200 | 1 | 22 | >125 | 11 | Deuser and Ross [1989] |
| 3200 | 2 | 13 | >37 | |||
| 12°25′N, 64°35′E | 2986 | 7.1 | >20 | 12 | Koppelmann et al. [2000] | |
| 12°25′N, 64°35′E | 2800–3000 | 22 | 20.73 | 12 | Haake et al. [1993] |
- a Numbers refer to the location (Loc.) as given in Figure 1. Honjo and Manganini [1993; 33°N, 22°W] and Haake et al. [1993] give planktic foraminiferal flux rates as part of bulk CaCO3 flux rate. Broerse et al. [2000] refer to the coccolith and calcareous dinophyte flux rate, allowing for estimates of the planktic foraminiferal test flux rate.
[6] Flux rates of planktic foraminiferal CaCO3 are calculated from the multinet material according to the species-specific and size-specific settling velocity [Takahashi and Bé, 1984], test weight, and test concentration [cf. Schiebel and Hemleben, 2000]. As the vertical heave velocity of the multinet (0.5 m s−1) is much higher than the sinking velocity of tests (0.001–0.018 m s−1), the sampled test assemblage is regarded as being at steady state. According to the depth distribution of live specimens and empty tests, the flux rate data provided here are based on the total planktic foraminiferal assemblage from the depth interval of 80–100 m and below. Flux rate data refer to the lower depth limits of the MCN hauls. For calculation of CaCO3 flux rates from the sediment trap material, including planktic foraminiferal specimens smaller than 100 μm, the weight of small specimens is calculated according to the allometric relation w = a + exp(c(x − b)) where w (μg) denotes the weight of tests and x (μm) denotes test diameter, with a = −0.54, b = 73.88, and c = 0.00116 (n = 9, r2 = 0.997).
3. Planktic Foraminiferal Population Dynamics
3.1. Living Fauna
[7] In the eastern North Atlantic (Table 1: BIOTRANS), low planktic foraminiferal abundance in late January and early February is assumed to represent the overall winter situation (Figure 2 and Table 4). The first increase in individual numbers occurs during March at depths of 50–100 m, which is well displayed by the distribution of Globigerina bulloides (Figure 3), and possibly results from the enhanced availability of food. Maximum frequency in spring results from wind-driven water mixing, improved feeding conditions, and increased growth rates in the upper water column [Schiebel et al., 1995]. Due to mass sedimentation and scavenging, shallow-dwelling species also occur below the seasonal thermocline (Figure 2a). During spring, the so-called deep-dwelling species ascend to shallow waters and do not contribute to the stock of cytoplasm-bearing specimens below 500 m. In contrast, during late spring and summer (May through August), deep-dwelling G. scitula and G. hirsuta live below 100 m and are a major part of the subsurface fauna. During summer (June–July), highest standing stocks occur in the upper 40–60 m, and Neogloboquadrina incompta and Turborotalita quinqueloba are major components of the shallow dwelling fauna. Large numbers of living and dead specimens in the fall result from improved feeding conditions due to redistribution of chlorophyll to surface waters, entrainment of nitrate, and increased primary production in the mixed layer [Schiebel et al., 2001]. During October, “aphotic” conditions occur, feeding conditions decline, and planktic foraminiferal production decreases.
[8] Strong seasonality also prevails in the Arabian Sea (Table 2: WAST), where the highest planktic foraminiferal numbers are found during the late NE monsoon (March) and during the SW monsoon (June–September) (Figure 4). During the monsoon seasons, the food availability for planktic foraminifers at WAST is improved compared with the intermonsoonal periods due to upwelling off Oman and resulting filaments that move toward the open ocean.
[9] Most planktic foraminifers are restricted to the upper 60–80 m [cf. Berger, 1969; Watkins et al., 1996; Hemleben et al., 1989] and to the upper 200 m under exceptional circumstances, e.g., periods of wind-driven deep mixing [Schiebel et al., 1995]. Cytoplasm-bearing specimens that are transported below their normal habitat by turbulent mixing can no longer subsist due to a lack of food or low radiation (in the case of symbiont bearing species), and are part of the vertical test flux. Below 100–200 m water depth, the living:dead ratio (cytoplasm bearing specimens versus empty tests) decreases exponentially from ∼10 to ∼0.1 at 1500–2000 m depth. Emerson et al. [1997] assume that significant amounts of empty, sinking tests are not present in the upper ocean. So-called deep-dwelling species such as G. truncatulinoides are found below 100 m, or at least below the mixed layer, during most of the year [cf. Hemleben et al., 1989]. However, deep-dwelling species are much less abundant than shallow-dwelling species [Bé, 1977].
3.2. Concentration of Empty Tests in the Water Column
[10] Increased numbers of empty and sinking tests generally result from increased growth rates in the surface waters. The largest empty-test numbers within the upper 2500 m during spring at BIOTRANS (Figure 2b) result from mass sedimentation [cf. Anderson and Sarmiento, 1994]. An isolated, empty-test patch at 1000–1500 m during July August (Figure 2b) is formed mainly by small (100–125 μm) and slowly sinking tests of T. quinqueloba. A disproportionately large number of T. quinqueloba tests is also observed during September, when this species constitutes about 50% of the sediment trap assemblage (Figure 5) while T. quinqueloba makes up only 5–15% of the living fauna and 10% of the surface sediment assemblage.
[11] It is conceivable that small tests (e.g., of T. quinqueloba) accumulate in mesobathyal waters at BIOTRANS due to deceleration as a result of increasing viscosity of the seawater. The viscosity of seawater increases with increasing salinity and decreasing temperature and pressure [Dietrich et al., 1975]. At BIOTRANS, the viscosity of seawater may increase as a result of decreasing temperature down to 1500 m [cf. van Aken, 2000]. Increased viscosity between 700 and 1200 m may also be linked to enhanced salinity, indicating the presence of Mediterranean Sea outflow water (MSW) that was repeatedly revealed by CTD measurements during the sampling (Table 1). Below the MSW, decreased viscosity causes increased velocity of sinking tests and the accumulation of small tests dissipates.
[12] At WAST, the number of empty tests in the deep-water column is much higher than at BIOTRANS (Figures 2 and 4), although planktic foraminiferal standing stocks in surface waters are similar at both sites during the SW monsoon and during spring, respectively. This discrepancy may be due to the better preservation of settling tests within the oxygen minimum zone (OMZ) of the Arabian Sea [cf. Hermelin, 1992] than in the well-oxygenated water column of the eastern North Atlantic.
4. Fluxes of Planktic Foraminiferal Tests and CaCO3
4.1. Differential Test and CaCO3 Flux Rates
[13] As a consequence of ecological and autecological prerequisites, planktic foraminiferal test and calcite fluxes display complex, intermittent pulses [e.g., Sautter and Thunell, 1989; Bijma et al., 1994]. According to sediment trap investigations, the highest CaCO3 flux rates at BIOTRANS (1000 m depth) are observed during spring when test production in the surface waters is highest (Figure 5) [cf. Honjo and Manganini, 1993]. With increasing depth, a shift of maximum flux rates from spring toward summer (July–August) takes place, reflecting the settling velocity of planktic foraminiferal tests. A comparison of CaCO3 and test flux rates reveals that during times of maximum CaCO3 flux rates (Figure 5a) only moderate test flux rates (Figure 5b) occur at 1000 m depth. In contrast, during June–July, the highest test flux rates occur when the CaCO3 flux rate at around 2000–2500 m is moderate. Maximum test flux rates at 2000 m during June–July (Figure 5b) are mainly caused by the small sized species T. quinqueloba. The difference between test and calcite flux rates indicates that comparatively few large tests dominate the spring CaCO3 flux pulse and that a large number of small tests, which settle through the water column at comparatively low velocity, constitute the CaCO3 flux pulse during the late summer [cf. Deuser, 1987]. The summer test pulse obtained by the sediment traps and multinet sampling (Figure 2b) mainly consists of tests 20–125 μm in size. Both large and small tests predominantly result from enhanced spring production.
4.2. Planktic Foraminiferal CaCO3 Flux Modes
[14] In the eastern North Atlantic at 47°N–57°N, maximum test production at depths >200 m during the spring bloom (Figures 6 and 7 and Table 4), causes highest flux rates within the upper 500 m, and distinct flux pulses in the deeper water column (Figure 7; ∼1500 m depth). During May, flux rates decrease but flux pulses occasionally still occur in the deeper water column. In the Azores region, low flux rates during August point toward oligotrophic conditions, and high flux rates and a distinct CaCO3 flux pulse at 1000 m depth during January are due to large numbers of G. truncatulinoides [Schiebel et al., 2002a, 2002b]. Comparatively, balanced flux rates in the Caribbean display a less distinct seasonality than in the high latitudes.
[15] Differential sinking velocities of planktic foraminiferal tests result in different settling tracks that can be traced through the water column (Figure 8). Following times of biological mass production in March–April and September, a vast number of large, quickly settling tests occurs in the deep-water column during May and October. At the same time, many small tests (100–125 μm) occur in deep waters that are scavenged by larger particles. Nevertheless, the majority of the small tests settle through the water column much more slowly than assumed from the test size and weight alone [cf. Takahashi and Bé, 1984]. Consequently, a “cloud” of slow-sinking small tests is present during summer (June–August) at around 1000 m depth (Figures 2 and 8). Deep-dwelling G. scitula contribute a minor part to the enhanced calcite budget at depths, and other deep-living species are virtually absent during summer at BIOTRANS. Slightly enhanced CaCO3 flux rates at 100–500 m during summer are caused by small sized and slow settling T. quinqueloba and N. incompta and have no significant impact on the test “cloud” at 1000 m depth. In autumn, increased production causes CaCO3 flux pulses and tests are scavenged from the “cloud” of tests [Schiebel et al., 2001], similar to the particle dynamics described by Thomsen and McCave [2000] from the bottom nepheloid layer.
[16] In the Arabian Sea, relatively high flux rates occur in the water column below 500 m depth, particularly in the upwelling area off Oman and in the southern Arabian Sea (Figure 6). At WAST, a seasonally pulsed CaCO3 flux is observed (Figure 9) corresponding to the late stages of the NE and SW monsoons during March and July September, respectively [cf. Rixen et al., 2000]. In the deep-water column, maximum planktic foraminiferal flux rates are delayed and were observed in April at 1500 m, and during August–September at 700–900 and 2500 m depth (Figure 9). The seasonally increased CaCO3 flux rates at WAST are much higher than at BIOTRANS, and likely result from better preservation of tests within the OMZ than in the well oxygenated water column, respectively.
4.3. Mass Sedimentation of Planktic Foraminiferal Tests
[17] Disproportionally high flux rates of >1 g CaCO3 m−2 d−1 at 1000–2500 m depth in the Arabian Sea occurred during March 1995 around the new moon (Figures 10 and 11, and Table 4). This CaCO3 flux pulse was mainly caused by large tests of G. sacculifer (>315–700 μm). Although G. sacculifer is frequent in the Arabian Sea [Auras-Schudnagies et al., 1989; Naidu and Malmgren, 1996; Conan and Brummer, 2000], mass sedimentation of large tests of G. sacculifer was observed only once during the study presented here. Production and flux of G. sacculifer are related to the synodic lunar cycle [Almogi-Labin, 1984; Bijma et al., 1990, 1994; Erez et al., 1991]. Therefore although the observed mass flux event does not display the average sedimentation (“steady particle rain”), mass sedimentation is characteristic of the deep-marine environment [cf. Anderson and Sarmiento, 1994], though under-represented in sediment trap and multinet samples. The temporal resolution of most deep traps (≥1000 m water depth) is too low to record single mass flux events. Shallow sediments traps (≤1000 m depth) have a low trapping efficiency [Scholten et al., 2001] and are not included here. Mass flux events may not be detected by sediment traps even if the test flux rates double or triple within a short time-interval, because the relatively small amount of large planktic foraminiferal tests yield no statistical significance. In contrast, the quantity of large tests obtained by multinet sampling is much larger than in trap samples and variation in standing stocks and flux rates is statistically significant. However, mass flux events are highly unpredictable and met only by chance with net hauls.
[18] Compared with the live fauna, surface sediments of the Arabian Sea and other tropical to subtropical ocean basins contain a disproportionately high portion of large tests [Peeters et al., 1999]. These tests may result from events like the mass sedimentation previously described. Pulsed flux events seem to yield a major contribution to the deep-sea sediment accumulation and remove bicarbonate on long-term timescales from the upper ocean and transfer it to deep-sea sediments [cf. Wefer, 1989; Berger and Wefer, 1990]. Mass sedimentation of large tests, however, requires the presence of species that have the autecological prerequisites to form large tests [Brummer et al., 1987; Hemleben et al., 1987; Caron et al., 1990] and that are adapted to specific ecologic conditions [Bijma et al., 1990; Huber et al., 2000; Kemp et al., 2000]. To understand such processes, detailed knowledge of population dynamics is crucial.
4.4. Planktic Foraminiferal CaCO3 Flux Rates
[19] Calcite flux rates determined from the total set of multinet and sediment trap data (Tables 1–3), including literature data (Table 5), range between <0.001 and >2000 mg m−2 d−1 (Figure 10). Flux rates span more than four orders of magnitude in each multinet depth interval, and the upper to lower quartile of flux rates covers about one order of magnitude (Figure 11). Flux rates within the upper 100 m of the water column are similar for each of the five investigated depth intervals and clearly result from complete vertical mixing. The most significant decrease in flux rates takes place between 100 and 700 m depths. Between 700 and 2500 m, and possibly below 2500 m, only small changes in flux rates occur (Figures 6, 7, and 11). Outliers with flux rates of >1000 mg m−2 d−1 between 1000 and 2500 m (Figure 11) result from mass sedimentation of G. siphonifera and G. sacculifer.
4.5. Dissolution of Planktic Foraminiferal Tests
[20] The most significant decrease in planktic foraminiferal test flux rates between 100 and 700 m takes place at depths where thermodynamic calcite dissolution does not occur [cf. Broecker and Peng, 1982]. This decrease in flux rates possibly results from increased bacterially mediated decomposition of cytoplasm and a decreasing pH in the microenvironments within foraminiferal tests [Schiebel et al., 1997b; cf. Turley and Stutt, 2000]. Bacterial activity is limited by oxygen, and bacterially mediated decomposition of cytoplasm is possibly less significant in low-oxygen environments than in well-oxygenated waters. According to Milliman et al. [1999], calcite dissolution far above the lysocline is probably biologically mediated, and takes place within the guts of grazers [cf. Jansen and Wolf-Gladrow, 2002] or is related to organic matter degradation. Both grazers and remineralization depend on oxygen and, therefore, calcite preservation is assumed to be better in low-oxygen environments, as demonstrated by differential flux modes in the North Atlantic and Arabian Sea (Figure 6). In the upwelling area off Oman and in the central and southern Arabian Sea, high flux rates of planktic foraminiferal tests within the prominent OMZ point toward a high degree of calcite preservation. The low oxygen content within the OMZ of Arabian Sea waters may permit limited decomposition of organic material and less dissolution of calcite than in the eastern North Atlantic. At 1000–1500 m depth, where the empty-test patch occurred in the eastern North Atlantic during July August (Figure 2), bacterially mediated dissolution of tests may be lower than above or may have already ceased.
[21] Dissolution of planktic foraminiferal tests within aggregates of marine snow is unlikely, although marine snow consists of organic matter and microbes, and contains planktic foraminiferal tests [Ransom et al., 1998]. Sedimentation of marine snow aggregates is probably too fast to allow for significant calcite dissolution (H. Jansen, University of Hamburg, written communication, October 2001). To conclude, dissolution of planktic foraminiferal tests in waters that are supersaturated with respect to calcite is hitherto not sufficiently explained.
4.6. Planktic Foraminiferal Flux Versus Pteropod, Calcareous Dinophyte, and Coccolith Flux
[22] The planktic foraminiferal, pteropod, and coccolith contribution to the total calcareous particle flux is assessed to estimate the composition of the total planktic carbonate budget (cf. Table 5). Fischer et al. [1996] state that each group, planktic foraminifers, coccoliths, and pteropods, constitutes about one-third of the total carbonate flux off Cape Blanc, West Africa [see also Kalberer et al., 1993]. High flux rates of pteropods seem to be restricted to distinct pulses with high temporal variability, indicating patchy distribution, and are not yet well understood [cf. Fischer et al., 1983; Wefer and Honjo, 1985; Fischer et al., 1996; Honjo, 1996]. On a global scale, pteropods account for ∼10% of the total planktic carbonate production [Fabry, 1990], and, due to high sinking velocity (R. Schiebel, unpublished data), most of the shells may arrive at the seafloor. In deep-sea sediments, pteropods are only sporadically present due to their dissolution susceptible, aragonitic shells [e.g., Almogi-Labin et al., 1986; Auras-Schudnagies et al., 1989].
[23] Calcareous dinophytes constitute probably only a minor part of the flux within the calcareous fine fraction in modern oceans [Broerse et al., 2000]. In the South Atlantic, calcareous dinophytes form up to 3.5% of deep-marine sediments (A. Vink, oral communication, Bremen University, 2001), the only number on modern sediment available to date.
[24] The proportion of planktic foraminifers and coccoliths within the total carbonate flux varies on large regional and temporal scales (Panama Basin [Honjo, 1982]; Equatorial Pacific [Dymond and Collier, 1988]; Sargasso Sea [Deuser et al., 1995]; Southern California [Ziveri et al., 1995]). Broerse [2000] estimates that coccoliths account for 4–38% of the total CaCO3 flux rate in the different ocean basins, with a global mean of only 12% [cf. Beaufort and Heussner, 1999]. Planktic foraminifers are assumed to constitute between 2 and 100% of the calcareous particle flux within different regions of the world's ocean (Table 5).
4.7. The Global Planktic Foraminiferal CaCO3 Flux
[25] The sample locations within the Atlantic, Caribbean, and Arabian Sea (Figure 1) cover a wide range of productivity regimes (Figure 12) [Berger et al., 1988; Berger, 1989; Antoine et al., 1996] and allow for a first-order estimate of the global planktic foraminiferal calcite budget. According to Berger [1989] and Milliman [1993], provinces of varying primary productivity and calcium carbonate sedimentation mainly differ with respect to latitude and individual hydrographic conditions (e.g., upwelling). An estimate of global planktic foraminiferal calcite flux rate is possible considering population dynamics, regional calcite flux rates, and hydrography. Regional variations in productivity and carbon export are identified on a global scale by Berger et al. [1988] and coincide well with the regional variability of planktic foraminiferal production and test flux rate (Figure 12).
[26] Detailed maps of estimated primary productivity created from satellite chlorophyll observations [Antoine et al., 1996] appear to be similar to the maps of vertically exported primary production given by Berger et al. [1988] and Berger [1989], although they are not linearly proportional and differ remarkably in detail. For example, in the upwelling area off Oman (Arabian Sea), the rate of primary productivity and exported primary production differ significantly, and the shallow (100 m) planktic foraminiferal CaCO3 flux rate is only weakly correlated with primary productivity (Figure 13). Therefore planktic foraminiferal CaCO3 flux rates are estimated according to the maps of Berger et al. [1988].
[27] To extrapolate from regional to global planktic foraminiferal flux rates, the absolute surface area of the global ocean is taken as 290 × 106 km2 [Milliman, 1993]. This number excludes areas that are devoid of living planktic foraminifers [Hemleben et al., 1989]. To estimate flux rates at different depth levels of the ocean, the numbers given by Menard and Smith [1966] and Dietrich et al. [1975] are applied.
[28] The average global planktic foraminiferal calcite flux rate at the 100-m depth-level is estimated to be 27.4 mg m−2d−1 (10 g m−2 yr−1), the arithmetic mean of all the available flux data (Figure 11). According to different global marine productivity and flux regimes [Berger, 1989] (Figure 12), the planktic foraminiferal CaCO3 flux rate ranges between 1.3 and 3.2 Gt (on average 2.9 Gt), which is 23–56% of the total global-marine “steady” calcite flux at 100 m depth based on the study of Milliman et al. [1999] and 6% of the total carbon flux following the work of Berger [1989] (Table 6). In the Caribbean, a tropical region of low seasonality, 23% of the exported planktic foraminiferal calcite arrives in the deep ocean (Table 4). In midlatitudes, where seasonal maximum production and flux rates occur during 2 months per year, and comparatively low production prevails during 10 months (Figures 2 and 8), the average portion of tests that settle on the seafloor amounts to ∼25%. During off-peak periods, 1–3% of the planktic foraminiferal test calcite that is produced in the surface ocean arrives at the seafloor (Table 6). Extraordinary flux pulses such as that described for G. sacculifer are likely to account for most of the planktic foraminiferal assemblage in deep-sea sediments. Such “mass dumps” (outliers in Figures 10 and 11) exceed the “off-peak sedimentation” and supply 31–77% to the formation of deep-marine carbonates (Table 6). On a global average, ∼25% of the total calcite produced by planktic foraminifers arrives in the deep ocean and at the sediment surface (Table 4 and Figure 14).
| Berger [1989] Total Carbon, Gt C yr−1 | Rel., % | This Study Planktic Foraminifers | Rel., % | Milliman et al. [1999] Total CaCO3, Gt CaCO3 yr−1 | ||
|---|---|---|---|---|---|---|
| Gt CCaCO3 yr−1 | Gt CaCO3 yr−1 | |||||
| Production | ||||||
| 100 m | 17.566– 32.062 | 5.8 | ||||
| Flux | ||||||
| 100 m | 2.602–6.421 | 6 | 0.157–0.389 | 1.315–3.240 | 23–56 | 5.8 |
| 500 m | 0.646–1.171 | 6 | 0.036–0.065 | 0.299–0.542 | ||
| 1000 m | 0.163–0.314 | 7–13 | 2.4 | |||
| 2500 m | 0.013–0.032 | 1–3 | ||||
| Mass dumps | 0.342–0.842 | 31–77 | ||||
| Seafloor | 1.1 | |||||
- a Data are given in gigatons (Gt) per year. Maximum and minimum flux rates were calculated using the upper and lower limits of the intervals given in the maps on primary productivity of Berger [1989] (see Figure 12). Relative numbers (Rel., %) refer to the absolute numbers to their left and right. Numbers for 2500 m and mass dumps are related to the amount of CaCO3 at the seafloor.
[29] Assuming that most of the planktic foraminiferal tests that arrive at 2500 m depth also reach the deep-seafloor (Figure 11; Table 4), the total planktic foraminiferal contribution of calcite to global deep-marine surface sediments amounts to about 0.355–0.874 Gt CaCO3 yr−1 (Table 6), which is 32–80% of the total CaCO3 accumulation on the seafloor (Figure 14) estimated at 1.1 Gt by Milliman [1974], Milliman and Droxler [1996], and Milliman et al. [1999]. The above mentioned estimates work out quite well under the assumption that the 1.1 Gt of CaCO3 given by Milliman and coworkers is correct. Considering the large range between the estimated minimum and maximum CaCO3 flux rates (Table 6), the calculated planktic foraminiferal CaCO3 budget is also in agreement with the lower estimation of Catubig et al. [1998] of 0.83 Gt CaCO3 yr−1.
[30] If coccoliths, pteropods, and dinophytes contribute an additional 12% [Beaufort and Heussner, 1999], <10% [Fabry, 1990], and 3.5% (Vink, oral communication, 2001), respectively (Figure 14), the budget would be at the upper level of the planktic foraminiferal carbonate contribution (∼75%). Assuming that coccoliths could possibly provide up to 38% to the global calcite budget [Broerse, 2000], the amount of coccoliths would amount to about two-thirds of that of planktic foraminifers.
5. Conclusions
[31] The planktic foraminiferal test flux is a consequence of the population dynamics, and the differential settling modes for different species are a precondition for the differences in the regional flux rates. The average planktic foraminiferal calcite flux rate at the 100-m depth-level is estimated to be 27.4 mg m−2 d−1 (10 g m−2 yr−1), 2.9 Gt CaCO3 yr−1 within the global pelagic ocean, which is 50% of the global-marine calcite of 5.8 Gt yr−1 [Milliman et al., 1999] and 6% of the total carbon flux [Berger, 1989]. Only 1–3% of the planktic foraminiferal test calcite that is produced in the surface ocean arrives at the seafloor under “off-peak conditions”, and “mass dumps” account for 31–77% of the planktic foraminiferal tests in deep-sea sediments. The most significant decrease in the planktic foraminiferal test flux rates between 100 and 700 m water depths possibly results from increased bacterially mediated decomposition of cytoplasm and a decreasing pH in microenvironments within foraminiferal tests. On a global average, ∼25% of the total calcite produced by planktic foraminifers arrives in the deep ocean and at the sediment surface. About 0.36–0.87 Gt planktic foraminiferal CaCO3 yr−1, which is 32–80% of the total 1.1 Gt CaCO3 [e.g., Milliman et al., 1999], accumulates on the seafloor.
Acknowledgments
[32] The masters, crews, and PIs of the cited expeditions of R/V Meteor, R/V Sonne, R/V Poseidon, and BO Arquipelago, are gratefully acknowledged for their support in sampling. The working groups of M. Alves (University of the Azores), J. Duinker, K. Lochte, and B. Zeitzschel (IfM Kiel), V. Ittekkot (GBC Hamburg), O. Pfannkuche (GEOMAR Kiel), and G. Wefer (Geo Bremen) generously provided samples and logistic help during all the stages of the projects. I especially thank Christoph Hemleben for supporting this work. Margret Bayer and a large team of colleagues and students at the laboratory in Tübingen are greatly appreciated for their assistance in the preparation of samples. For their critical comments on the manuscript, J. Casford, A. Hupe, H. Paul, P. Quinn, D. Schmidt, and B. Schmuker are gratefully acknowledged. For their helpful reviews, I thank H. Jansen, J. Milliman, and an anonymous reviewer. This research was funded as part of the Joint Global Ocean Flux Studies (JGOFS) on the North Atlantic and the Arabian Sea (German Minister of Education and Research, BMBF), and Canary Islands Açores Gibraltar Observations (CANIGO; MAST III).
