Description: Continental shelves and shelf seas play a central role in the global carbon cycle. However, their importance with respect to trace element and isotope (TEI) inputs to ocean basins is less well understood. Here, we present major findings on shelf TEI biogeochemistry from the GEOTRACES programme as well as a proof of concept for a new method to estimate shelf TEI fluxes. The case studies focus on advances in our understanding of TEI cycling in the Arctic, transformations within a major river estuary (Amazon), shelf sediment micronutrient fluxes and basin-scale estimates of submarine groundwater discharge. The proposed shelf flux tracer is 228-radium (T1/2 = 5.75 yr), which is continuously supplied to the shelf from coastal aquifers, sediment porewater exchange and rivers. Model-derived shelf 228Ra fluxes are combined with TEI/ 228Ra ratios to quantify ocean TEI fluxes from the western North Atlantic margin. The results from this new approach agree well with previous estimates for shelf Co, Fe, Mn and Zn inputs and exceed published estimates of atmospheric deposition by factors of approximately 3–23. Lastly, recommendations are made for additional GEOTRACES process studies and coastal margin-focused section cruises that will help refine the model and provide better insight on the mechanisms driving shelf-derived TEI fluxes to the ocean.

Description: A prominent two-step rise in atmospheric CO2 marked the end of the last glacial. The steps coincided with climatic intervals Heinrich Stadial 1 (HS1) and the Younger Dryas (YD). Records of 231Pa/230Th on sediment cores bathed by NADW, revealed a rapid reduction of the Atlantic Meridional Overturning Circulation (AMOC), during these intervals. It was argued that a weakened AMOC would have significantly reduced the efficiency of the biological pump and thus might have contributed to the rise in atmospheric CO2. Despite playing an important role, this process fails to account for the enigmatic drop in atmospheric Δ14C and δ13C during HS1 that marks the first step of the CO2-rise. Increasing CO2-concentrations with a simultaneous drop in their Δ14C, call for the ventilation of an old and 14C-depleted carbon reservoir. In this respect, several studies point to the presence of very old, 14C-depleted deep- waters in the glacial Southern Ocean, which rejuvenated during the last deglaciation. However, the accumulation of 14C-depleted, carbon-rich waters in the deep Southern Ocean requires circulation patterns that significantly differ from todays. Here we present a set of multi-proxy records to understand the evolution of the Southern Indian and the Southwest Pacific Oceans over the last 35,000 years. Our reconstructions are based on transects of four sediment cores from the South Indian and five sediment cores from the Southwest Pacific, covering the AAIW as well as the UCDW and LCDW. Our data show that throughout the last glacial the deep-water circulation of both Southern Ocean sectors weakened. This reduction favored the observed accumulation of 14C-depleted CO2 in Circumpolar Deep Waters (CDW) and allowed for the expansion of Antarctic ice sheets up to the continental shelf edge. Parallel to the HS1 increase of atmospheric CO2, the deep circulation picked up its pace and recovered toward the Holocene. This trend is in remarkable agreement with atmospheric changes in CO2, Δ14C and δ13C as well as the deglacial collapse of ice sheets. Hence, we are confident that the Southern Ocean played the dominant role in the first rise in atmospheric CO2. In addition the observed deglacial SPOC strengthening may have supported the transport of warm CDW onto the shelf areas since the timing of retreating Antarctic ice sheets is in good agreement with our recent reconstructions.