Inefficient burial of terrestrial organic carbon in deltas
Description
The world’s deltas are estimated to store ~58 Tg of terrestrial organic carbon (OCterr) annually, indicating their significant role in the global carbon cycle and atmospheric CO2. However, previous studies simply used a proto burial efficiency in the top layer of delta sediments to represent the OCterr burial efficiency in the whole delta, thus neglecting the effects of OCterr degradation after initial burial on the long-term OCterr burial efficiency in deltas. Here, we used a novel method to quantify sediment budgets and ages from the subaqueous Yellow River Delta. We demonstrate that different sediment pathways together with OCterr degradation result in a substantially, at least 50%, lower multi-decadal net OCterr burial efficiency than previous estimates. These novel results shed insights into the influence of sediment dynamics and OCterr degradation on the long-term OCterr burial. We propose considering time-scale and sediment pathway-dependent heterogeneity of OCterr burial in future studies. We therefore believe that the budget of OCterr buried in global deltas is currently overestimated and call for a re-visit to these budgets. Our data include ages, physical and chemical characteristics of surface and core sediments collected from the subaqueous Yellow River delta. Also included are the budgets of sediment delivery by the Yellow River, sediment deposition and removal in the subaqueous delta over the past decades.
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Daily water and sediment discharge data at the lowermost gauging station of the Yellow River together with the historical bathymetric data covering the whole subaqueous delta was collected from the Yellow River Conservancy Commission. Using the geospatial tool in Q-GIS, we converted the bathymetric data (lon., lat., depth) for each year through linear interpolation to a rectangular grid with a spatial resolution of 0.001°×0.001° (approximately 110m×110m). Intra-annual, annual, and multi-annual maps of depth-change and land gain (area above -0.5 m isobath) were generated by differencing bathymetry maps. Budgets of mass storage and wasting within a certain timespan were further estimated based on the depth-change map, using a sediment density of 1.36 g/cm3. We collected 33 surface sediment samples, 5 sub-cores (YR-2, YR-15, YR-21, YR-24, and YR- 26), and a gravity core (YR-38) from the subaqueous YRD during a survey cruise in August 2018 immediately after a monsoon flood. These sediments were analyzed for grain-size and surface area, respectively using a Malvern laser diffraction particle size analyzer (Mastersizer 3000) and a Quantachrome Nova 4200e. These samples were analyzed for total organic carbon (TOC) and total inorganic carbon (TIC) using a Perkin-Elmer 2400 CHNS Analyzer. Total carbon (TC) content and TOC were measured separately before and after acidification with 10% hydrochloric acid (HCl). Samples were measured for Δ14Corg and OC stable isotopes (δ13Corg) composition respectively using an accelerator mass spectrometry (AMS) facility and a Thermo 253 Plus Isotope Ratio Mass Spectrometer (IRMS) coupled with an Elemental Analyzer at Qingdao National Laboratory for Marine Science and Technology - China. Activities of 210Pb, 226Ra and 137Cs in YR-38 core sediments were measured by a well-type HPGe gamma-spectrometry (GSW275L) with an energy resolution of 1.8 keV (at 1332 keV). An endmember method based stable isotopic composition δ13Corg was used to differentiate between terrestrial and marine contributions of organic carbon to the total organic carbon in the sediment. We further used a combined δ13Corg and Δ14Corg three-endmember model based on Monte Carlo simulation approach to calculate the relative contributions of modern OC, pre-aged soil OC, and fossil OC in the deltaic sediments. The burial efficiency (BE) of OCterr in sediment is determined by the ratio between the OCterr loading of the sediment and the input loading by the river (OCriver): BE = 100% × OCterr loading/OCriver loading