Tatsuhiko Sakamoto

Temperature and salinity variations of Mediterranean Sea surface waters over the last 16,000 years from records of planktonic stable oxygen isotopes and alkenone unsaturation ratios

Alkenone unsaturation ratios and planktonic d18O records from sediment cores of the Alboran, Ionian and Levantine basins in the Mediterranean Sea show pronounced variations in paleo-temperatures and -salinities of surface waters over the last 16,000 years. Average sea surface temperatures (SSTs) are low during the last glacial (averages prior to 13,000 years: 11‐15°C ), vary rapidly at the beginning of the Holocene, and increase to 17‐18° Ca t all sites during S1 formation (dated between 9500 and 6600 calendar years). The modern temperature gradient (2‐ 3°C ) between the Mediterranean sub-basins is maintained during formation of sapropel S1 in the Eastern Mediterranean Sea. After S1, SSTs have remained uniform in the Alboran Sea at 18°C and have fluctuated around 20°C in the Ionian and Levantine Basin sites. The d18O of planktonic foraminifer calcite decreases by 2‰ from the late glacial to S1 sediments in the Ionian Basin and by 2.8‰ in the Levantine Basin. In the Alboran Sea, the decrease is 1.7‰. Of the 2.8‰ decrease in the Levantine Basin, the eVect of global ice volume accounts for a maximum of 1.05‰ and the temperature increase explains only a maximum of 1.3‰. The remainder is attributed to salinity changes. We use the temperature and salinity estimates to calculate seawater density changes. They indicate that a reversal of water mass circulation is not a likely explanation for increased carbon burial during S1 time. Instead, it appears that intermediate and deep water formation may have shifted to the Ionian Sea approximately 2000 years before onset of S1 deposition, because surface waters were as cold, but saltier than surface water in the Levantine Basin during the Younger Dryas. Sapropel S1 began to form at the same time, when a significant density decrease also occurred in the Ionian Sea.

The sapropel record of the eastern Mediterranean Sea — results of Ocean Drilling Program Leg 160

Research on sediments recovered during Ocean Drilling Leg 160 has concentrated on two issues: the first concerned the stratigraphy of sapropel formation, the second was oriented to clarify specific processes that explain sapropel origin. Progress has been made in the construction of stratigraphic composites out of sedimentary sequences from individual holes at each of the palaeoceanographic sites. On the composites, initial work has resulted in the establishment of high-resolution and intermediate-resolution stratigraphies for three sites (963, 964, 967); correlation of sedimentary cycles to astronomical (insolation) cycles extends the stratigraphies to Sites 969 and 966. The sapropel occurrences in the marine and land sequences over the entire Eastern Mediterranean are correlated; with the resolution that can be obtained from isotope studies, groups of sapropels occurred simultaneously over the entire basin. In detail, however, the temporal and facies patterns of sapropel sequences differ between individual sites and depositional basins. The differences may be related to effects of water depth, diagenesis, and post-depositional tectonic attenuation of sequences. Studies on the geochemistry and facies of sapropels agree that anoxic conditions favoured preservation of organic matter in sapropels, caused the enrichment of trace metals associated with sapropels, and helped to preserve primary sedimentary structures. Besides, all evidence is consistent with elevated fluxes of organic matter and associated elements during sapropel events.

Eastern Mediterranean surface water temperatures and d18O composition during deposition of sapropels in the late Quaternary

0indices) and d 18 O of planktonic foraminifer calcite (d 18 Ofc) across late Pleistocene sapropel intervals show that d 18 Ofc decreased (between 1 and 4.6%) and SST increased (between 0.7� and 6.7� C). Maximal d 18 Oseawater depletion of eastern Mediterranean surface waters at the transition is between 0.5 and 3.0%, and in all but one case exceeded the depletion seen in a western Mediterranean core. The depletion in d 18 Oseawater is most pronounced at sapropel bases, in agreement with an initial sudden input of monsoon-derived freshwater. Most sapropels coincide with warming trends of SST. The density decrease by initial freshwater input and continued warming of the sea surface pooled fresh water in the surface layer and prohibited deep convection down to ageing deep water emplaced during cold and arid glacial conditions. An exception to this pattern is ‘‘glacial’’ sapropel S6; its largest d 18 Oseawater depletion (3%) is almost matched by the depletion in the western Mediterranean Sea, and it is accompanied by surface water cooling following an initially rapid warming phase. A second period of significant isotopic depletion is in isotope stage 6 at the 150 kyr insolation maximum. While not expressed as a sapropel due to cold SST, it is in accord with a strengthened monsoon in the southern catchment. INDEX TERMS: 1055 Geochemistry: Organic geochemistry; 1620 Global Change: Climate dynamics (3309); 4267 Oceanography: General: Paleoceanography; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: Mediterranean Sea, sapropels, sea surface temperatures, oxygen isotopes, Quaternary Citation: Emeis, K.-C., et al., Eastern Mediterranean surface water temperatures and d 18 O composition during deposition of sapropels

Reconstruction of paleoproductivity in the Sea of Okhotsk over the last 30 kyr

[1] Marine- and terrestrial-derived biomarkers (alkenones, brassicasterol, dinosterol, and long-chain n-alkanes), as well as carbonate, biogenic opal, and ice-rafted debris (IRD), were measured in two sediment cores in the Sea of Okhotsk, which is located in the northwestern Pacific rim and characterized by high primary productivity. Down-core profiles of phytoplankton markers suggest that primary productivity abruptly increased during the global Meltwater Pulse events 1A (about 14 ka) and 1B (about 11 ka) and stayed high in the Holocene. Spatial and temporal distributions of the phytoplankton productivity were found to be consistent with changes in the reconstructed sea ice distribution on the basis of the IRD. This demonstrates that the progress and retreat of sea ice regulated primary productivity in the Sea of Okhotsk with minimum productivity during the glacial period. The mass accumulation rates of alkenones, CaCO3, and biogenic opal indicate that the dominant phytoplankton species during deglaciation was the coccolithophorid, Emiliania huxleyi, which was replaced by diatoms in the late Holocene. Such a phytoplankton succession was probably caused by an increase in silicate supply to the euphotic layer, possibly associated with a change in surface hydrography and/or linked to enhanced upwelling of North Pacific Deep Water. INDEX TERMS: 1050 Geochemistry: Marine geochemistry (4835, 4850); 1055 Geochemistry: Organic geochemistry; 4267 Oceanography: General: Paleoceanography; KEYWORDS: Okhotsk Sea, paleoproductivity, sediment Citation: Seki, O., M. Ikehara, K. Kawamura, T. Nakatsuka, K. Ohnishi, M. Wakatsuchi, H. Narita, and T. Sakamoto (2004), Reconstruction of paleoproductivity in the Sea of Okhotsk over the last 30 kyr, Paleoceanography, 19, PA1016,

Decreased surface salinity in the Sea of Okhotsk during the last glacial period estimated from alkenones

[1]xa0Studies of sediment cores from the Sea of Okhotsk, which is characterized by seasonal sea ice, have shown a large variability of sea ice indicators during the glacial-interglacial cycles. In this study, we apply the relative abundance of the C37:4 alkenone to total C37 alkenones (%C37:4) as a molecular indicator of salinity and water masses to further investigate surface oceanographic condition in the Sea of Okhotsk in the glacial period. We found a large fluctuation of %C37:4 (4–35%) with higher values (20∼35%) during the last glacial period and lower values (<8%) during warm periods, suggesting a decreased surface salinity in the glacial period. The variation of %C37:4 was found to be consistent with the ice rafted debris (IRD) in the sediment core (correlation coefficient (r2) between %C37:4 and IRD: 0.72). This suggests that the duration of seasonal sea ice was longer in the glacial period than today. The lowered surface salinity in the glacial might also have been seriously affected by the close-off of the shallow Soya Strait, which currently transports massive volumes of saline water from the Sea of Japan to the Sea of Okhotsk.