摘要:

Several explanations for the 200 to 280 ppm glacial/interglacial change in atmospheric CO2concentrations deal with variations in southern ocean phytoplankton productivity and the related use or nonuse of major plant nutrients. An hypothesis is presented herein in which arguments are made that new productivity in today's southern ocean (7.4 × 1013g yr−1) is limited by iron deficiency, and hence the phytoplankton are unable to take advantage of the excess surface nitrate/phosphate that, if used, could result in total southern ocean new production of 2−3 × 1015g C yr−1. As a consequence of Fe‐limited new productivity, Holocene interglacial CO2levels (preindustrial) are as high as they were during the last interglacial (≈ 280 ppm). In contrast, atmospheric dust Fe supplies were 50 times higher during the last glacial maximum (LGM). Because of this Fe enrichment, phytoplankton growth may have been greatly enhanced, larger amounts of upwelled nutrients may have been used, and the resulting stimulation of new productivity may have contributed to the LGM drawdown of atmospheric CO2to levels of less than 200 ppm. Background information and arguments in support of this hypothesis ar

ISSN:0883-8305    

DOI:10.1029/PA005i001p00001

年代:1990

数据来源: WILEY

摘要:

We have perturbed a simple model of oceanic circulation in a series of sensitivity tests to determine how circulation changes associated with the last glacial maximum could affect observable or potentially observable paleoceanographic parameters. Ultimately, we want to use such paleoceanographic data to construct models of glacial ocean circulation. We have found that the different circulation patterns that we imposed in our sensitivity tests have dramatically different responses for the distribution of PO4, O2, and ∂13C contents of the oceans. They also produce different levels of low‐latitude productivity. By comparing the coupled responses of several of these paleoceanographic parameters to the responses from the sensitivity tests it is possible to constrain the possible glacial circulation scenarios to a relatively small number. We give, as one example, a case where intermediate turnover is significantly higher than the present and deep ocean turnover has dropped. In addition, exchange of waters between the intermediate‐ and high‐latitude surface ocean is much lower. The result of this scenario is an atmosphere with 60 ppm lower atmospheric CO2, low latitudes with about 40% higher productivity, and a change in the planktic/benthic ∂13C contrast of about 0.3‰. When we compare our model to other independent paleoceanographic data, we find that the major point of disagreement is in the distribution of surface water ∂13C in the Antarctic region. We believe that somewhat more complex box models which take open system effects into account will resolve thi

ISSN:0883-8305    

DOI:10.1029/PA005i001p00015

年代:1990

数据来源: WILEY

摘要:

Variations in the contribution of North Atlantic Deep Water (NADW), relative to North Pacific Deep Water (NPDW), to the Southern Ocean, are assessed by comparing δ13C records from the mid‐depth North Atlantic, deep Southern Ocean, and deep equatorial Pacific Ocean. In general, the relative contribution of NADW was greater during interglaciations than glaciations of the past 550,000 years. An increase in the NADW flux to the Southern Ocean since the last glaciation was proposed to have resulted in higher atmospheric CO2in the Holocene (Broecker and Peng, 1989). Glacial‐interglacial variations in the proportion of NADW in the Southern Ocean may have also influenced atmospheric CO2levels over the past 550,000 years. The greatest relative flux of NADW to the Southern Ocean occurred during interglacial stage 11. Faunal data suggest that the North Atlantic polar front and southern Indian Ocean subtropical convergence zone were located farthest poleward during stage 11. Warmth in these locations and a strong southward flux of NADW during stage 11 may be causally linked by the NADW formation process/warm water return route (Gordon, 1986). Time series analysis indicates that δ13C variations in the deep Southern Ocean occur at the same frequencies as the Earth's orbital variations and are coherent and in phase with δ18O. At most, 50% of the glacial‐interglacial δ13C amplitude in the Southern Ocean is due changes in the contribution of NADW. The remainder is probably due to mean ocean δ1

ISSN:0883-8305    

DOI:10.1029/PA005i001p00043

年代:1990

数据来源: WILEY

摘要:

This work identifies features of the life history of the deep‐living planktonic foraminiferGloborotalia truncatulinoidesthat offer possibilities for reconstructing characteristics of the past ocean's main thermocline. Part 1 considers two aspects of variation in shell size: the predictability of growth‐related size changes, which we need to calculate changes in shell chemistry, and a model for environmentally forced changes in shell size frequency, which suggests how shell size may be a direct source of paleoceanographic information. Even though size variation among shells ofG. truncatulinoidesat the same stage of growth (same number of chambers) exceeds the size difference between successive growth stages, growth‐related size changes are predictable because shells tend to grow at the same rate. Variation in initial shell size is sufficient to account for the large size variation found among shells at the same stage of growth. Several lines of evidence suggest thatG. truncatulinoidesreproduces at ∼600 m in the water column, and it appears that vertical mixing to that depth is required to return juvenile shells to the surface. Anything that prevents this, such as shallowing of the thermocline, can interfere withG. truncatulinoides'life history in such a way that its shell size‐frequency distributions are altered. Among shells falling to the seafloor from a reproducing population are large numbers of both juvenile and adult sizes and, if reproduction is inhibited, the frequencies of those sizes are reduced and the relative frequency of intermediate‐sized shells increases. A paleoceanographically useful result of this is that the topography of the main thermocline tends to be reflected in the size‐frequency distribution ofG. truncatulinoides'shells from the underlying sediments: In areas where the top of the main thermocline is shallower than ∼600 m, production of juveniles (125–177 µm) and reproductive adults (>425 µm) is reduced, leaving predominantly intermediate‐size

ISSN:0883-8305    

DOI:10.1029/PA005i001p00055

年代:1990

数据来源: WILEY

摘要:

The sea surface paleotemperature of the South China Sea (SCS) is estimated quantitatively, based on the quantitative analysis of planktonic foraminiferal faunas recovered from piston cores V36‐06‐3, V36‐06‐5, and SO49‐KL8 on the northern continental slope. The results lead to a reconstruction of the history of paleocirculation patterns during the last 200,000 years. The postglacial and interglacial transbasinal circulation pattern was replaced by a semienclosed basin flow pattern during glacial episodes. Lowered sea level transformed the SCS into a semienclosed basin with the only passageway to the open ocean in its northeastern corner. The East Asian Monsoon, which governs circulation patterns in the SCS both today and during the late Quaternary, created both clockwise (summer) and counterclockwise (winter) surface gyres during glacials. The inflow of tropical surface water‐masses from both the Pacific and Indian oceans decreased or was completely cut off from this marginal sea, causing a greater decrease in glacial sea surface temperature than in the open ocean of the same latitude. Inferred increased influence of Pacific Temperate Waters and decreased influence of Tropical‐Subtropical Waters are confirmed by changes in the planktonic foraminiferal assemblages. Glacial‐interglacial fluctuation in sea surface temperature (SST) (6.8–9.3°C for winter and 2–3°C for summer), as well as the seasonality (4–6°C for postglacial and interglacials and 9–10°C for glacials), are much greater than the adjacent open western Pacific, reflecting the amplification of the climatic

ISSN:0883-8305    

DOI:10.1029/PA005i001p00077

年代:1990

数据来源: WILEY

摘要:

Quantitative study of climatically sensitive nannofossils in the sediments of the Gulf of Aden has provided a climatic and oceanographic scenario for the northwestern Indian Ocean during the late Neogene. The surface water temperature was relatively cold during the latest middle Miocene to late Miocene (10.7 to 6.32 Ma) with a probable warm interval in the early late Miocene (9.82 to 8.49 Ma). This cooling is attributed to increased upwelling in the northwestern Indian Ocean. The surface water was relatively warm from the latest late Miocene to the middle of the late Pliocene (6.32 to 2.59 Ma). The most intense cooling took place during the late Pliocene to early Pleistocene (2.59 to 1.69 Ma). Intense upwelling during this interval coincides with worldwide climatic deterioration. A probable short warm interval (1.69 to 1.59 Ma) followed this cold interval. In the Pleistocene the surface water was cold from 1.59 to 1.00 Ma and from 0.61 to 0.24 Ma and warm from 1.00 to 0.61 Ma and after 0.24 Ma.

ISSN:0883-8305    

DOI:10.1029/PA005i001p00091

年代:1990

数据来源: WILEY