PP51C-1343 0800h
Freshwater input into the Gulf of Mexico Prior to the 8.2 cal kyr BP Cool Event in Greenland Ice Core Records
The most prominent event recorded in Greenland ice core records over the past 10 calendar kiloyears before present (cal kyr BP) is an abrupt cooling at 8.2 cal kyr BP, which lasted for 300-400 yrs. It has been proposed that this climatic event was caused by a weakening of the thermohaline circulation in the North Atlantic forced by a vast outflow of freshwater through the Hudson Strait from glacial lakes Agassiz and Ojibway. Sediment core MD02-2550 from Orca Basin located in the northern Gulf of Mexico (GOM) provides an early Holocene record (10 to 7 cal kyr BP) of GOM climate changes, input from the Mississippi River system and adjacent continental areas. Paired analysis of Mg/Ca, a proxy of sea surface temperatures (SSTs), and \delta$^{18}$O on the planktonic foraminifera { \it Globigerinoides ruber } (white variety; 250 -350 $\mu$m) sampled at 0.5 cm (providing $\sim$20 year resolution), indicate a large isotopic excursion of $\sim$ -1 $\permil$ \delta$^{18}$O seawater from 8.5 to 8.4 cal kyr BP, coincident with the drainage of glacial lakes Agassiz and Ojibway at 8.47 cal kyr BP. We consider three possible sources for this freshwater pulse: freshwater input from glacial lakes, meltwater input from the drainage of the Laurentide Ice Sheet's final dome, and/or increased precipitation in North American drainage basins. Mg/Ca-SSTs show minimal mean change across the freshwater interval and an average of $\sim$28 $\deg$C from 7 to 10 cal kyr BP, which is within the current mean SSTs for GOM ($\sim$28 $\deg$C). Mg/Ca-SST and \delta$^{18}$O seawater time series contain concentrations of variance near 500, 220, 146, and 60 years, significant at the 90 % confidence level, indicating possible sensitivity to solar variations. Ongoing faunal assemblage work will provide additional assessment for SST, sea surface salinity (SSS) and nutrient changes during the early Holocene in the GOM.
PP51C-1344 0800h
Changing Water Vapor Transport Across the Panama Isthmus: A Role for the Tropics in Global Climate Variability?
Westward transport of freshwater across the Panama Isthmus helps to maintain the salinity contrast between the Atlantic and Pacific Oceans, important in global thermohaline circulation and climate. Relatively low sea-surface salinities and a strong, shallow pycnocline in the northeastern tropical Pacific warm pool region reflect high net precipitation. New oxygen isotope and Mg/Ca data suggest that this region of the tropics has experienced significant changes in temperature and salinity over the last glacial cycle. A comparison of these records to paleoclimate archives from the Caribbean Sea, the Atlantic Ocean, and Greenland suggest that there have been significant changes in cross-isthmus vapor transport associated with deep ocean circulation and high latitude temperature changes, providing convincing evidence of a strong climatic linkage between the tropics and the high latitudes.
PP51C-1345 0800h
Sea Surface Salinity reconstruction in Fiji during the last century from Multi-Proxies of Coral Skeleton using Neural Network: preliminary results.
In contrast to the Equatorial eastern pacific where sea surface temperature (SST) anomalies are maximum during an ENSO, the western tropical pacific is essentially affected by sea surface salinity (SSS) variability which could be a powerful indicator of ENSO during the past. Coral skeleton is regarded as the best archives to record oceanic conditions over the last centuries. Unfortunately, up to now, it seems difficult to decipher SST and SSS reconstructions even from multi proxies time series. Midway Tahiti and Darwin, Fiji islands are key areas to record SSS. Located at the southern warm pool edge, this zone is submitted to heavy precipitation due to South Pacific Convergence Zone (SPCZ) during La Niña and salty subtropical waters are advected westward during El Niño. This situation has been well documented from salinity data collected by ships between 1976 and 2000, but SSS measurements are sparse in historical database, specially in this area. By combining geochemists and biologists understanding we recognize the processes involved in the coral skeleton deposit (cf BG13). We infer that the response to environmental forcing is embedded with biological reactions and geochemical records are not linear in the time. This result is obtained by using a neural network on multiproxies (carbon and oxygen isotopic ratio, trace elements and density) to reconstruct SSS and SST during the last century in a coral head collected in Yasawa (Fiji). The proxies necessary for estimating SSS and SST are chosen by heuristic variable selection (HVS); SSS is reconstructed from 4 proxies while for SST 7 proxies are used. Thus, the first step of this mathematical treatment is to train our data set during the period 1961-1997. Then, the reconstructions are derived. Over the last century, the interannual SSS and SST variability's respectively range from 1% to 1°C. A comparison with SOI shows that as it has been observed from the last decades, during the whole XXth century, SSS is higher and SST lower during El Niño. The SSS increase is due to reduced precipitation and also the displacement of cold and salty water by a zonal advection.
PP51C-1346 0800h
N. Atlantic Salinity Change during the Last Century: Surface forcing at the salinity maximum propagated to the central Caribbean by subsurface waters
Using multi-proxy records of salinity ($\delta^{18}$O) and temperature (Sr/Ca, $\delta^{18}$O) in the skeletons of sclerosponges, we have extended the instrumental records that suggest a recent adjustment in N. Atlantic salinity. Changes in salinity calculated using recent calibrations in sclerosponges between temperature and Sr/Ca and $\delta^{18}$O, agree very well with the recent salinity increases of 0.3 documented by Curry et al. [2003] between the 1950's and 1990's. The sclerosponge data however indicate a continuous increase in salinity due to the increased heat availability of the earth's surface is less significant than salinity fluctuations related to the decadal scale forcing of the NAO. If these subsurface salinity records are related to surface forcing at the salinity maximum of the subtropical N. Atlantic where dry air masses from the Sahara cause very high net evaporation, one would expect the Bahamas records to be representative of a regional response as these waters form the shallow subsurface waters of much of the Caribbean. The Salinity Maximum surface Waters (SMW) of the N. Atlantic play a crucial role in the thermohaline circulation and the meridional transport of heat as they contribute to both the NADW and the subtropical-tropical meridional overturning cells (MOC's). Although the SMW source area of these subsurface waters is broad and diffuse, the salinity and temperature records from the Bahamas as well as previously published Sr/Ca and $\delta^{18}$O records from Jamaica [Haase-Schramm et al., 2003] suggest a common N. Atlantic source but there are slight differences between the records such as stronger secular trends in the central Caribbean. However, comparison of subsurface sclerosponge salinity records in both locations indicate a regional salinity response to increased availability of heat to the oceans. Whether this response is a deepening of the mixed layer, a change in surface forcing of subsurface water masses, or a combination to the two remains enigmatic. References Curry, R., B. Dickson, and I. Yashahaev, A change in the freshwater balance of the Atlantic Ocean over the past four decades, Nature, 426, 826-829, 2003. Haase-Schramm, A., F. B\H{o}hm, A. Eisenhauer, and W. C. Dullo, Sr/Ca ratios and oxygen isotopes from sclerosponges: temperature history of the Caribbean mixed layer and thermocline during hte Little Ice Age, Paleoceanography, 18, doi: 10.1029/2002PA000830, 2003.
PP51C-1347 0800h
Centennial-Scale Changes in Tropical North Atlantic Salinity Inferred from Scleractinian Corals
Although there exist reasonably good sea-surface temperature records for most portions of the surface oceans over the past 100 years, there are poor records of salinity variations over the same time period. Salinity variations are important because when combined with temperature they govern the density of ocean water and hence oceanic circulation. Using the well established relationships between the oxygen isotopic composition of coral skeletons and temperature, it is possible to utilize the temperature records to remove this influence from the oxygen isotopic composition and thereby extract salinity records. We have applied this method to corals collected from the Cape Verde Islands in the eastern tropical Atlantic Ocean and from the Lesser Antilles in the western tropical Atlantic and compared the results to sclerosponges from the sub-tropical North Atlantic. The Cape Verde Islands are positioned at a critical juncture in the tropical Atlantic. They are located in an area which shows the highest correlation with the temperature of the north tropical Atlantic Ocean (TNA) and one where there is a steep salinity gradient associated with the presence of the sub-tropical Atlantic gyre. Salinity changes in this area are not associated with input of freshwater from either precipitation or runoff, but rather from changes in evaporation and migration of the sub-tropical Atlantic gyre. Recent work by others has suggested the presence of secular changes in the salinity of surface waters in the tropical and subtropical North Atlantic. In particular, the Salinity Maximum Waters (SMW) of the North Atlantic, generally located between 15°-30° N and 20°-50° W, have reportedly increased in salinity over the last 50-100 years. The reported increase is slight, and does not apply to the entire Atlantic, but it represents a significant change in the freshwater input, or evaporation-precipitation (E-P) balance over large sections of the Atlantic. Our data for the eastern tropical North Atlantic is based on oxygen isotopic records from three specimens of the species Siderastrea sp. from the island of Sal in the Cape Verde Islands collected in 2002. All records show poor correlation to the intra-annual temperature changes, but show similar decadal variations which between 1940 and 2000 show significant correlation to the North Atlantic Oscillation (NAO). Such correlations are similar to relationships between calculated salinity and the NAO observed in the skeletons of sclerosponges from the sub-tropical North Atlantic.
PP51C-1348 0800h
Seasonal Sea Surface Temperature and Salinity Changes Over Last 22 kyr in the Western Arabian Sea
The variation of oxygen isotopic ratios between individual shells of planktic foraminifera of a given species and size may provide a novel out look on paleoceanography and foram ecology. In this context, the oxygen isotope analyses of individual Globigerinoides sacculifer were carried out from the Ocean Drilling Program Site 723A in the western Arabian Sea to unravel the seasonal changes for last 22 kyr. Analyses of single shells of G.sacculifer show ranges of 0.54 to 2.0 per mil in $\delta$$^{18}$O at various levels covering a time span of last 22 kyr. Maximum inter-shell $\delta$$^{18}$O variability and high standard deviation is noticed from 22 to 10 kyr, where as from 10 kyr onwards the inter shell $\delta$$^{18}$O variability decreased. The individual contribution of sea surface temperature (SST) and sea surface salinity (SSS) on the inter shell $\delta$$^{18}$O values of G. sacculifer were quantified. Seasonal SST ranges from 0.1 to 2.6 degree centigrade with maximum seasonal SST change during last glacial period and minimum seasonal SST change from 12 to 0.5 kyr. Similarly, greater SSS changes are documented during the last glacial period than in Holocene. The higher seasonal SST and SSS changes were caused due to more westerly winds during the last glacial period. On the other hand, lower seasonal SST and SSS changes during the Holocene were driven by less westerly winds. The seasonal SST and SSS data suggest that the change over phase from the strong westerly winds to less westerly winds took place between 12 to 10 kyr in the western Arabian Sea.
PP51C-1349 0800h
Implications for Reconstructions of the Upper Ocean Stratification From Core-top Planktonic Foraminiferal Multispecies \delta$^{18}$O and Mg/Ca
As one of the most important features of the upper tropical Atlantic the depth of the seasonally varying thermocline increases from east to west as a function of tropical wind strength and oceanic heat transport. Upwelling in the east due to easterly trade winds, and equatorial divergent flow bring cool, nutrient-rich intermediate depth water to the surface, therefore creating steep vertical thermal gradients. The wind-induced westward flow of surface currents allows tropical warmer surface water to pile up in western parts of the basin beyond 30$^\circ$ W, resulting in the westward dip of the thermocline and the seasonally scarcely changing Western Atlantic Warm Water Pool. To monitor the variability in surface hydrography and vertical temperature gradients in tropical oceans, planktonic foraminifera are one of the most commonly used tools in paleoceanography. They live vertically dispersed in the upper water column, recording in their tests \delta$^{18}$O and Mg/Ca the ambient \delta$^{18}$O composition and temperature of the surrounding sea water. In the geological record, oxygen isotopes provide information about the calcification temperature and calcification depth of one species relative to another, usually without information on the absolute vertical depth of calcification, because for most species calcification depth varies depending on hydrography. Here, we present combined \delta$^{18}$O and Mg/Ca measurements of 9 planktonic foraminiferal species from 65 tropical sediment core-top samples spread from the E- to W-Atlantic and the Caribbean Sea. The chosen transect from about 15$^\circ$ S and 10$^\circ$ E to 20$^\circ$ N and 75$^\circ$ W follows the South Equatorial Current and the North Brazil Current and crosses the Caribbean Sea. Comparison of the isotopic and trace metal data, measured on the same organisms and mineral phases, with modern hydrographic data enable us to trace characteristic patterns of the modern upper ocean stratification and to estimate the absolute depth of calcification of planktonic foraminiferal species. However, the comparison of sediment surfaces with recent hydrography is a strict test of the usefulness of a paleoceanographic tool.
PP51C-1350 0800h
North Atlantic Intermediate-Depth Circulation During the Holocene: Evidence From Simultaneous Benthic Foraminiferal Mg/Ca and Cd/Ca Data
Until just recently, the Holocene epoch had been considered a time of relative climate stability. However, a growing body of evidence is beginning to emerge that suggests centennial-millennial scale variability in both the North Atlantic atmospheric circulation and deepwater circulation (O'Brien et al., 1995; Bond et al., 1997; Bianchi and McCave, 1999; Oppo et al., 2003). At present, intermediate-water variability during the Holocene is not well understood, yet the formation of intermediate water masses in the North Atlantic may be an important mechanism of northward heat transport. We present a down-core record of benthic foraminiferal Cd/Ca and Mg/Ca from an intermediate-depth core taken from the Little Bahama Bank. Our data suggest that the influence of North Atlantic Intermediate Water at this site varied on centennial-millennial timescales, providing further evidence of ocean/atmosphere variability during the Holocene epoch.