Description: New records of stable silicon isotope signatures (δ30Si) together with concentrations of biogenic opal and organic carbon from the central (9° S) and northern (5° S) Peruvian margin reveal changes in diatom productivity and nutrient utilization during the past 20,000 years. The findings are based on a new approach using the difference between the δ30Si signatures of small (11-32μm) and large (〉150μm) diatom fractions (Δ30Sicoscino-bSi) in combination with the variance in diatom assemblages for reconstruction of past upwelling intensity. Combination of our records with two previously published records from the southern upwelling area off Peru (12-15° S) shows a general decoupling of the environmental conditions at the central and southern shelf mainly caused by a northward shift of the main upwelling cell from its modern position (12-15° S) towards 9° S during Termination 1. At this time only moderate upwelling intensity and productivity levels prevailed between 9° S and 12° S interpreted by a more northerly position of Southern Westerly Winds and the South Pacific Subtropical High. Furthermore, a marked decrease in productivity at 12-15° S during Heinrich Stadial 1 coincided with enhanced biogenic opal production in the Eastern Equatorial Pacific, which was induced by a southward shift of the Intertropical Convergence zone and enhanced northeasterly trade winds. Modern conditions were only established at the onset of the Holocene. Past changes in preformed δ30Si signatures of subsurface waters reaching the Peruvian Upwelling System did not significantly affect the preserved δ30Si signatures.

Description: The development of stable barium (Ba) isotope measurements provides a novel tool to investigate the geochemical cycling of Ba in the ocean and its sediments. In sediment pore waters, gradients of dissolved Ba concentrations result from various diagenetic processes. The distribution and fractionation of Ba isotopes in the pore waters are expected to further improve our understanding of these early diagenetic control mechanisms. Here, we present pore water profiles of dissolved stable Ba isotopic signatures (δ138Bapw) from shallow water sediments covering the entire Pearl River Estuary (PRE) in Southern China. We find pronounced depth-dependent Ba isotope variations generally showing a shift from heavy to light δ138Bapw signatures from the sediment surface down to 15 cm depth. These gradients are well reproduced by a diffusion-reaction model, which generates an apparent fractionation factor (138ε) of −0.60 ± 0.10‰ pointing to preferential removal of low-mass Ba isotopes from the pore water during solution-solid phase interactions. Consequently, the combined diagenetic processes induce the highest δ138Bapw values of +0.5 to +0.7‰ in the pore waters of the topmost sediment layer. Although the detrital fraction dominates the Ba content in the PRE surface sediments, the determined gradients of pore water Ba isotopes, together with concentration variations of Ba and other redox-sensitive elements such as manganese (Mn), show that non-detrital excess Ba carriers including Mn oxides and authigenic barite clearly affect the post-depositional Ba dynamics. Stable Ba isotopes are thus a potentially powerful tracer of Ba geochemistry during early sediment diagenesis in estuarine depositional environments. Key Points We present a data set of dissolved stable Ba isotopic compositions in surface sediment pore waters of a large river estuary Pore water Ba isotope values generally decrease with increasing sediment depth, reflecting post-depositional Ba isotope fractionation A diffusion-reaction model predicts the distribution and fractionation of stable Ba isotopes in the sediment pore waters well

Description: During most of the year, diatom production in the ice-covered Central Arctic Ocean (CAO) is limited by light availability and nutrient supply. Therefore, biological production is thought to be generally low, with higher biological production at the sea ice edge and over partially ice-free shelf areas. The major surface ocean current in the CAO is the Transpolar Drift (TPD), which transports sea ice and water from the rivers and shelves of the Laptev and the East Siberian Seas across the CAO toward the Fram Strait, carrying high amounts of terrestrial-derived material over long distances. We used Si isotopes (δ30Si) to better understand the difference between lower and higher biological production areas and how the TPD potentially affects the Si cycle in the CAO. Our data show low dissolved Si concentrations ([DSi]) paired with high values of δ30Si-DSi in all surface samples indicating fractionation by diatoms. Specifically, outside the TPD influence, all nutrients were depleted and supply was limited due to stratified conditions, thus preventing further phytoplankton growth in the area during the sampling time in late summer-early fall. In contrast, under the TPD influence, diatom primary production was limited by low nitrate and strongly limited by light due to the presence of sea ice, even though [DSi] values were much higher than outside the TPD. Based on δ30Si, we could identify low but measurable DSi utilization in the TPD, potentially highlighting the importance of sea ice-attached diatoms transported to the CAO via the TPD for the Si cycle in this region. Key Points - Primary production and silicon utilization outside the Transpolar Drift are higher than under its influence due to more light availability - Primary production and lateral water transport under the Transpolar Drift influence were identified from silicon isotope composition - The Transpolar Drift delivers high dissolved silicon to the surface Arctic Ocean, a unique feature not seen in any other open ocean