Geochemsitry of minerals and glass shards at DSDP Leg 67 Holes

During Leg 67, the Middle-America Trench transect off Guatemala was drilled across the convergent margin of southern Mexico and Central America south of the Tehuantepec Ridge. The data of Leg 66, north of the Tehuantepec Ridge, and that of Leg 67 provided the opportunity to establish a continuous chronology of airborne volcanic ashes intercalated within the sediments (Aubouin et al., 1979; von Huene et al., 1980). Sites of both expeditions are favorably located for obtaining a good record of the explosive volcanicity of these areas, given the proximity of the volcanic sources and the position of the sites under the prevailing winds.

Planktic and benthic 14C reservoir ages, calibrated by a suite of 14C plateaus in the glacial-to-deglacial Suigetsu atmospheric 14C record

We here present a compilation of planktic and benthic 14C reservoir ages for the Last Glacial Maximum (LGM) and early deglacial from 11 key sites of global ocean circulation in the Atlantic and Indo-Pacific Ocean. The ages were obtained by 14C plateau tuning, a robust technique to derive both an absolute chronology for marine sediment records and a high-resolution record of changing reservoir/ventilation ages (Delta14C values) for surface and deep waters by comparing the suite of planktic 14C plateaus of a sediment record with that of the atmospheric 14C record (Sarnthein et al., 2007, doi:10.1029/173GM13). Results published thus far used as atmospheric 14C reference U/Th-dated corals, the Cariaco planktic record, and speleothems (Fairbanks et al., 2005, doi:10.1016/j.quascirev.2005.04.007; Hughen et al., 2006, doi:10.1016/j.quascirev.2006.03.014; Beck et al., 2001, doi:10.1023/A:1008175728826). We have now used the varve-counted atmospheric 14C record of Lake Suigetsu terrestrial macrofossils (Ramsey et al., 2012, doi:10.1126/science.1226660) to recalibrate the boundary ages and reservoir ages of the seven published records directly to an atmospheric 14C record. In addition, the results for four new cores and further planktic results for four published records are given. Main conclusions from the new compilation are: (1) The Suigetsu atmospheric 14C record on its varve counted time scale reflects all 14C plateaus, their internal structures and relative length previously identified, but implies a rise in the average 14C plateau age by 200-700 14C yr during LGM and early deglacial times. (2) Based on different 14C ages of coeval atmospheric and planktic 14C plateaus, marine surface water Delta14C may have temporarily dropped to an equivalent of ~0 yr in low-latitude lagoon waters, but reached >2500 14C yr both in stratified subpolar waters and in upwelled waters such as in the South China Sea. These values differ significantly from a widely assumed constant global planktic Delta14C value of 400 yr. (3) Suites of deglacial planktic Delta14C values are closely reproducible in 14C records measured at neighboring core sites. (4) Apparent deep-water 14C ventilation ages (equivalents of benthic Delta14C), deduced from the sum of planktic Delta14C and coeval benthic-planktic 14C differences, vary from 500 up to >5000 yr in LGM and deglacial ocean basins.

Geochemistry of sediments cores in the Mediterranean Sea

We investigated five time-equivalent core sections (180-110 kyr BP) from the Balearic Sea (Menorca Rise), the easternmost Levantine Basin and southwest, south, and southeast of Crete to reconstruct spatial patterns of productivity during deposition of sapropels S5 and S6 in the Mediterranean Sea. Our indicators are Ba, total organic carbon and carbonate contents. We found no indications of Ba remobilization within the investigated core intervals, and used the accumulation rate of biogenic Ba to compute paleoproductivity. Maximum surface water productivity (up to 350 g C/m2/yr) was found during deposition of S5 (isotope stage 5e) but pronounced spatial variability is evident. Coeval sediment intervals in the Balearic Sea show very little productivity change, suggesting that chemical and biological environments in the eastern and western Mediterranean basins were decoupled in this interval. We interpret the spatial variability as the result of two different modes of nutrient delivery to the photic zone: riverderived nutrient input and shoaling of the pycnocline/nutricline to the photic zone. The productivity increase during the formation of S6 was moderate compared to S5 and had a less marked spatial variability within the study area of the eastern Mediterranean Sea. Given that S6 formed during a glacial interval, glacial boundary conditions such as high wind stress and/or cooler surface water temperatures apparently favored lateral and vertical mixing and prevented the development of the spatial gradients within the Eastern Mediterranean Sea (EMS) observed for S5. A non-sapropel sediment interval with elevated Ba content and depleted 18O/16O ratios in planktonic foraminifer calcite was detected between S6 and S5 that corresponds to the weak northern hemisphere insolation maximum at 150 kyr. At this time, productivity apparently increased up to five times over surrounding intervals, but abundant benthic fauna show that the deep water remained oxic. Following our interpretation, the interval denotes a failed sapropel, when a weaker monsoon did not force the EMS into permanent stratification. The comparison of interglacial and glacial sapropels illustrates the relevance of climatic boundary conditions in the northern catchment in determining the facies and spatial variability of sapropels within the EMS.

Sedimentology of ODP Leg 104 holes

The long-term record of glacial/interglacial cycles indicates three major paleoceanographic regimes in the Norwegian Sea. The period since the first major glaciation over Scandinavia at 2.56 Ma is characterized by high-frequency, low-amplitude oscillations of ice-rafted debris inputs, a lowered salinity, and decreased carbonate shell production in surface waters as well as overall strong carbonate dissolution at the sea floor. These conditions indicate a more zonal circulation pattern in the Northern Hemisphere and a relative isolation of surface and bottom waters in the Norwegian Sea. The generally temperate glacial climate was only interrupted by episodic weak intrusions of warm Atlantic waters. These intrusions have been detected in considerable magnitude only at Site 644, and thus are restricted to areas much closer to the Norwegian shelf than during earlier periods. The interval from 1.2 to 0.6 Ma is characterized by an increase in carbonate shell production and a better preservation, as well as a change in frequency patterns of ice-rafted debris inputs. This pattern reflects increasing meridionality in circulation-strengthening contrasts in the Norwegian Sea between strong glaciations and warm interglacials. The past 0.6 Ma reveal high-amplitude oscillations in carbonate records that are dominated by the 100-k.y. frequency pattern. Glacial/interglacial sedimentary cycles in the ODP Leg 104 drill sites reveal a variety of specific dark lithofacies. These dark diamictons reflect intense iceberg rafting in surface waters fed by surges along the front of marine-based parts of the continental ice sheets in the southeastern sector of the Norwegian Sea and are associated with resuspension of reworked fossil organic carbon and strong dissolution at the sea floor. Piling up of huge iceberg barriers along the Iceland-Faeroe-Scotland Ridge might have partially blocked off surface water connections with the North Atlantic during these periods