Sediment geochemistry PASOM research cruise (2009) - Murray Ridge region, northern Arabian Sea

In this study, we investigate phosphorus (P) and iron (Fe) cycling in sediments along a depth transect from within to well below the oxygen minimum zone (OMZ) in the northern Arabian Sea (Murray Ridge). Pore-water and solid-phase analyses show that authigenic formation of calcium phosphate minerals (Ca-P) is largely restricted to where the OMZ intersects the seafloor topography, likely due to higher depositional fluxes of reactive P. Nonetheless, increased ratios of organic carbon to organic P (Corg/Porg) and to total reactive P (Corg/Preactive) in surface sediments indicate that the overall burial efficiency of P relative to Corg decreases under the low bottom water oxygen concentrations (BWO) in the OMZ. The relatively constant Fe/Al ratio in surface sediments along the depth transect suggest that corresponding changes in Fe burial are limited. Sedimentary pyrite contents are low throughout the ~25 cm sediment cores at most stations, as commonly observed in the Arabian Sea OMZ. However, pyrite is an important sink for reactive Fe at one station in the OMZ. A reactive transport model (RTM) was applied to quantitatively investigate P and Fe diagenesis at an intermediate station at the lower boundary of the OMZ (bottom water O2: ~14 µmol/L). The RTM results contrast with earlier findings in showing that Fe redox cycling can control authigenic apatite formation and P burial in Arabian Sea sediment. In addition, results suggest that a large fraction of the sedimentary Ca-P is not authigenic, but is instead deposited from the water column and buried. Dust is likely a major source of this Ca-P. Inclusion of the unreactive Ca-P pool in the Corg/P ratio leads to an overestimation of the burial efficiency of reactive P relative to Corg along the depth transect. Moreover, the unreactive Ca-P accounts for ~85% of total Ca-P burial. In general, our results reveal large differences in P and Fe chemistry between stations in the OMZ, indicating dynamic sedimentary conditions under these oxygen-depleted waters.

Sedimentary and structural controls on seismogenic slumping within mass transport deposits from the Dead Sea basin

GIA acknowledges funding from the Carnegie Trust to undertake fieldwork for this project. SM acknowledges the Israel Science Foundation (ISF grant no. 1436/14) and the Ministry of National Infrastructures, Energy and Water Resources (grant no. #214-17-027). RW was supported by the Israel Science Foundation (ISF grant no. 1245/11). We thank Hugo Ortner and Pedro Alfaro for careful and constructive reviews.

GDGT and n-alkane data of sediment core GeoB4901-8

The tight coupling between the atmospheric and oceanic circulation in the equatorial Atlantic region makes this area an important region for paleoclimatic research. Previous studies report the occurrence of large amounts of terrigenous material and soil organic carbon (SOC) within the marine sediments of the eastern Gulf of Guinea. We use the accumulation rates (AR) of branched glycerol dialkyl glycerol tetraethers (GDGTs) to identify variations in SOC delivery to the Niger Fan over the last 35 ka, and compare these records to long-chain n-alkanes as a proxy for higher plant material, to an inorganic proxy for terrigenous input (aluminum AR) and to indicators for the marine productivity (AR of carbonate and crenarchaeol). In addition, sea surface temperatures (SSTs) are calculated based on the TEX86H index and environmental factors affecting the SST-reconstructions are discussed. Our results indicate that Al AR are closely connected to the rate of mean sea level change after 15 ka BP, with an additional influence of the increased monsoonal precipitation and extended vegetation cover corresponding to the African Humid Period (14.8-5.5 ka BP). Branched GDGT AR appears to be determined by shelf erosion in addition to the interplay of monsoonal precipitation and vegetation cover controlling soil erosion. Long-chain n-alkane concentrations clearly show a different trend than the other proxies, which might be due to their predominant eolian transport. Paleo-SSTs show a clear shift from colder temperatures during the last glacial period (20-22 °C) to warmer temperatures during the Holocene (24-26 °C). However, TEX86H-based SSTs are cold-biased compared to recent SSTs and Mg/Ca-based SST reconstructions, which is probably caused by a high seasonality of the Thaumarchaeota, with a maximum productivity of these organisms during the cold summer months. However, a sub-surface production of GDGTs and/or a potential bias of SST reconstruction by terrestrial input could not be completely excluded.

(Table 1) AMS 14C dates from sediment cores RC12-10 and GYRE97-6PV-20

Proxy records from two piston cores in the Gulf of Mexico (GOM) provide a detailed (50-100 year resolution) record of climate variability over the last 14,000 years. Long-term (millennial-scale) trends and changes are related to the transition from glacial to interglacial conditions and movement of the average position of the Intertropical Convergence Zone (ITCZ) related to orbital forcing. The d18O of the surface-dwelling planktic foraminifer Globigerinoides ruber show negative excursions between 14 and 10.2 ka (radiocarbon years) that reflect influx of meltwater into the western GOM during melting of the Laurentide Ice Sheet. The relative abundance of the planktic foraminifer Globigerinoides sacculifer is related to transport of Caribbean water into the GOM. Maximum transport of Caribbean surface waters and moisture into the GOM associated with a northward migration of the average position of the ITCZ occurs between about 6.5 and 4.5 ka. In addition, abundance variations of G. sacculifer show century-scale variability throughout most of the Holocene. The GOM record is consistent with records from other areas, suggesting that century-scale variability is a pervasive feature of Holocene climate. The frequency of several cycles in the climate records is similar to cycles identified in proxy records of solar variability, indicating that at least some of the century-scale climate variability during the Holocene is due to external (solar) forcing.

14C specific radiocarbon ages and bulk ages accompanied by geochemical proxys from surface sediment samples in the Black Sea

Transfer of organic carbon (OC) from the terrestrial to the oceanic carbon pool is largely driven by riverine and aeolian transport. Before transport, however, terrigenous organic matter can be retained in intermediate terrestrial reservoirs such as soils. Using compound-specific radiocarbon analysis of terrigenous biomarkers their average terrestrial residence time can be evaluated. Here we show compound-specific radiocarbon (14C) ages of terrigenous biomarkers and bulk 14C ages accompanied by geochemical proxy data from core top samples collected along transects in front of several river mouths in the Black Sea. 14C ages of long chain n-alkanes, long chain n-fatty acids and total organic carbon (TOC) are highest in front of the river mouths, correlating well with BIT (branched and isoprenoid tetraether) indices, which indicates contribution of pre-aged, soil-derived terrigenous organic matter. The radiocarbon ages decrease further offshore towards locations where organic matter is dominated by marine production and aeolian input potentially contributes terrigenous organic matter. Average terrestrial residence times of vascular plant biomarkers deduced from n-C29+31 alkanes and n-C28+30 fatty acids ages from stations directly in front of the river mouths range from 900 ± 70 years to 4400 ± 170 years. These average residence times correlate with size and topography in climatically similar catchments, whereas the climatic regime appears to control continental carbon turnover times in morphologically similar drainage areas of the Black Sea catchment. Along-transect data imply petrogenic contribution of n-C29+31 alkanes and input via different terrigenous biomarker transport modes, i.e., riverine and aeolian, resulting in aged biomarkers at offshore core locations. Because n-C29+31 alkanes show contributions from petrogenic sources, n-C28+30 fatty acids likely provide better estimates of average terrestrial residence times of vascular plant biomarkers. Moreover, sedimentary n-C28 and n-C30 fatty acids appear clearly much less influenced by autochthonous sources than n-C24 and n-C26 fatty acids as indicated by increasing radiocarbon ages with increasing chain-length and are, thus, more representative as vascular plant biomarkers.