Organic carbon chemistry of late Tertiary to Quaternary sediments ODP Leg 107

We analyzed samples from ODP Holes 652A and 654A (Leg 107, Tyrrhenian Sea) for the amount, type, and thermal maturity of organic matter. The sediments encompass clastic and biogenic lithologies, which were deposited on the passive margin east of Sardinia since the late Miocene to the Pleistocene. Marine, hypersaline/evaporitic, lacustrine/riverine, and finally hemipelagic marine conditions with occasional anoxic(?) interludes gave rise to very diverse sedimentary facies. The majority of samples is lean in organic matter (<0.2% TOC). Notable exceptions are Tortonian sediments (TOC average 0.3%), Messinian oil shales from Core 107-652A-64R (up to 11% TOC), Messinian lacustrine/fluvial sediments from Hole 652A (TOC average 0.42%,), and Pleistocene sapropel samples (>2% TOC). The Messinian oil shale in Hole 652A appears to be the only mature hydrocarbon source rock. In general, Pliocene sediments are the leanest and least mature samples. Pleistocene and Pliocene samples derive organic matter from a marine source. In spite of obvious facies differences in the Messinian between the two sites, pyrolysis results are not conclusive in separating hypersaline facies of Site 654 from the fresh water facies of Site 652, because both appear to have received terrestrial organic tissue as the main component of TOC. It is apparent from the distribution of maximum pyrolysis temperatures that heat flow must have been considerably higher at Site 652 on the lower margin in the Messinian. Molecular maturity indices in lipid extracts substantiate the finding that the organic matter in Tortonian and Messinian samples from Hole 654A is immature, while thermal maturation is more advanced in coeval samples from Hole 652A. Analyses of lipid biomarkers showed that original odd-even predominance was preserved in alkanes and alkylcyclohexanes from Messinian samples in Hole 654A, while thermal maturation had removed any odd-even predominance in Hole 652A. Isomerization data of hopanes and steranes support these differences in thermal history for the two sites. Hopanoid distribution further suggests that petroleum impregnation from a deeper, more mature source resulted in the co-occurrence of immature and mature groups of pentacyclic biomarkers. Even though the presence of 4-methylsteranes may imply that dinoflagellates were a major source for organic matter in the oil shale interval of Hole 652, we did not find intact dinoflagellates or related nonskeletal algae during microscopic investigation of the organic matter in the fine laminations. Morphologically, the laminations resemble bacterial mats.

Age model, stable isotopes, major element proportions and endmember unmixing results from four western tropical Atlantic cores

We investigate changes in the delivery and oceanic transport of Amazon sediments related to terrestrial climate variations over the last 250 ka. We present high-resolution geochemical records from four marine sediment cores located between 5 and 12° N along the northern South American margin. The Amazon River is the sole source of terrigenous material for sites at 5 and 9° N, while the core at 12° N receives a mixture of Amazon and Orinoco detrital particles. Using an endmember unmixing model, we estimated the relative proportions of Amazon Andean material ("%-Andes", at 5 and 9° N) and of Amazon material ("%-Amazon", at 12° N) within the terrigenous fraction. The %-Andes and %-Amazon records exhibit significant precessional variations over the last 250 ka that are more pronounced during interglacials in comparison to glacial periods. High %-Andes values observed during periods of high austral summer insolation reflect the increased delivery of suspended sediments by Andean tributaries and enhanced Amazonian precipitation, in agreement with western Amazonian speleothem records. Increased Amazonian rainfall reflects the intensification of the South American monsoon in response to enhanced land-ocean thermal gradient and moisture convergence. However, low %-Amazon values obtained at 12° N during the same periods seem to contradict the increased delivery of Amazon sediments. We propose that reorganizations in surface ocean currents modulate the northwestward transport of Amazon material. In agreement with published records, the seasonal North Brazil Current retroflection is intensified (or prolonged in duration) during cold substages of the last 250 ka (which correspond to intervals of high DJF or low JJA insolation) and deflects eastward the Amazon sediment and freshwater plume.

Halogen deposition in polar snow and ice

The atmospheric chemistry of iodine and bromine in polar regions is of interest due to the key role of halogens in many atmospheric processes, particularly tropospheric ozone destruction. Bromine is emitted from the open ocean but is enriched above first-year sea ice during springtime bromine explosion events, whereas iodine is emitted from biological communities hosted by sea ice. It has been previously demonstrated that bromine and iodine are present in Antarctic ice over glacial-interglacial cycles. Here we investigate seasonal variability of bromine and iodine in polar snow and ice, to evaluate their emission, transport and deposition in Antarctica and the Arctic and better understand potential links to sea ice. We find that bromine enrichment (relative to sea salt content) and iodine concentrations in polar ice do vary seasonally in Arctic snow and Antarctic ice and we relate such variability to satellite-based observations of tropospheric halogen concentrations. Peaks of bromine enrichment in Arctic snow and Antarctic ice occur in spring and summer, when sunlight is present. Iodine concentrations are largest in winter Antarctic ice strata, contrary to contemporary observations of summer maxima in iodine emissions.