摘要:

The CO2record for air bubbles from the Byrd Station ice core suggest a drawdown in the ocean‐atmosphere carbon reservoir during the early Holocene. Such a drawdown would require a corresponding increase in the CO3=ion concentration in the deep sea. We report here the results of a search in Atlantic sediments for evidence that the lysocline showed a corresponding deepening. While both the pteropod and the calcite preservation records we have obtained are consistent with expectation, they are not conclusiv

ISSN:0883-8305    

DOI:10.1029/93PA00423

年代:1993

数据来源: WILEY

摘要:

A comparison of the deglaciation δ18O record of northeastern Atlantic core SU 81‐18 with that calculated by adding the δ18O changes due to sea surface temperature and ice volume variations shows that planktonic foraminifera have also recorded seawater δ18O changes due to salinity (local advection and evaporation‐precipitation (E‐P)) changes. Using a simple ocean box model of meltwater dilution during the deglaciation, we show that sea surface salinity covaried with temperature. Despite the injection of meltwater associated with the climate warming, sea surface salinity was high when sea surface water was warm and low during cold events. This implies that the combined effect of local E‐P changes and northward advection of saline subtropical water were the main factor responsible for the surface water salinity variations in the northeastern Atlantic Ocean during the last 18

ISSN:0883-8305    

DOI:10.1029/93PA00455

年代:1993

数据来源: WILEY

摘要:

We analyzed a 2‐year sediment trap record of 76 species of Radiolaria from three locations across the northern California Current System. Q‐mode factor analysis identifies the fundamental trends in the radiolarian data with a five‐factor model that explains 86% of the trap data. The factor assemblages that emerge from this analysis have temporal and spatial aspects that reflect fundamental oceanographic features of the eastern North Pacific. The most dominant process revealed by this analysis is related to the seasonality of the California Current. Two factors illustrate this: the California Current factor, which has highest loadings during summer and fall, and the Winter factor, which is most important in the winter and spring. Together they account for 75% of the information in the trap samples from the two moorings located in the coastal region extending to ∼300 km offshore. The oceanic environment sampled by the mooring 650 km offshore is strongly influenced by transitional waters and those from the Central and Subarctic Gyres. Loadings of the three factors related to these oceanographic regions show marked differences between years, which illustrates the latitudinal variability in the divergence of the North Pacific Current. Spatial distributions of the trap factors in surface sediments beneath the California Current System generally agree with the temporal and spatial patterns they exhibit in the traps. Nonetheless, the trap factor model can only describe up to 50% of the radiolarian sediment data, which means the species ratios in the sediment traps are significantly different than those in the sediments. A factor model using only robust species reveals the same basic trends as the first analysis, but the culled model is able to describe a larger fraction of the sediment data (63%–91% in sediments directly beneath the mooring sites). However, the increase in the amount of sediment data explained by the culled model is not distributed equally across all factors but is due to very high estimates of one factor in the coastal sediments, the California Current factor. At the Nearshore mooring site (130 km offshore) this factor explains 69% of the sediment data versus only 32% of the data in the traps where it was derived. This discrepancy suggests that either these two years of trap data are anomalous and have underestimated the predominance of the California Current assemblage over long time periods or the culled factor model, rather than eliminating the dissolution bias, actually overemphasizes the importance of well‐preserved species in the coastal

ISSN:0883-8305    

DOI:10.1029/93PA00384

年代:1993

数据来源: WILEY

摘要:

Downcore diatom data from a deep basin in Granite Harbor, Antarctica, provide new, high‐resolution information concerning changes in oceanographic and depositional processes that have occurred during approximately the last 1250 years. The influence of global‐scale climatic change, during the time of the Medieval Warm Period and possibly the Little Ice Age, is revealed by long‐term changes in floral assemblages, as recorded by changes in the relative concentration ofNitzschia cylindrus. The diatom data suggest that Granite Harbor experienced less annual ice and pack ice cover during both these periods, possibly because of higher temperatures during the Medieval Warm Period and stronger offshore winds during the Little ice Age. The sedimentary record reveals several distinct, short‐lived events, in which nearly monospecific blooms ofCorethron criophilumandChaetocerosspp. occurred, possibly the result of strong stabilization of the upper water column and were most likely a response to regional, rather than global conditions. Although short‐lived, these events may play a significant role in the delivery of organic carbon and biogenic silica to the

ISSN:0883-8305    

DOI:10.1029/93PA00561

年代:1993

数据来源: WILEY

摘要:

Evolutionary events within early PleistoceneGephyrocapsaassemblages provide at least four biostratigraphically useful events occurring in the Matuyama Chron after the Olduvai Subchron. We establish an informal taxonomy based on three size groups (5.5 µm) resulting in four reproducible biostratigraphic events that reflect episodes of adaptive radiation within theGephyrocapsalineage. Four additional, late Pliocene‐early Pleistocene events are also investigated. The successful astronomical calibration of oxygen isotope stratigraphies from Deep Sea Drilling Project site 607 and Ocean Drilling Program site 677 in the Matuyama Chron permits calibration of the biostratigraphic events to these uniquely resolved isotope chronologies. Shackleton et al.'s [1990] time scale gives the following age estimates (in million years): (reemG) reentrance of medium‐sizedGephyrocapsa(1.028, 0.957) (677, 607); (tlG) top largeGephyrocapsa(1.240, 1.227); (blG) base largeGephyrocapsa(1.457, 1.479); (bmG) base medium‐sizedGephyrocapsa(1.670, 1.700); (tHS) topH. sellii(1.470, 1.241); (tCM) topC. macintyrei(1.597, 1.640); (tDB) topD. brouweri(1.950) (607 only); (baDT) base acmeD. triradiatus(2.153) (607 only). Further comparison with other, less resolved, ∂18O stratigraphies indicate that event reemG is diachronous by 71 kyr, consistently being younger in the mid‐latitudes. Event tlG appears synchronous, clustering in stage 37 in all sites. Event blG is slightly older (22 kyr) in the mid‐latitudes than in the equatorial Pacific. Also, event bmG is older in the mid‐latitudes (30 kyr). Event tHS shows the largest difference, occurring 229 kyr later at site 607, corresponding to 12 isotope stages. The critical paleoisotherm separating “young” from “old” last occurrences ofH. selliiwas probably located between ∼11°N and 18°N in the Atlantic Ocean. EventtCMis clearly time transgressive, occuring 43 kyr earlier at site 607. EventtDBoccurred a few kiloyears wi

ISSN:0883-8305    

DOI:10.1029/93PA00755

年代:1993

数据来源: WILEY