Depth, estimated age, and associated geochemical information from sediment core PRGL 1-4

The terrigenous sedimentary budget of passive margin records variations in past sedimentary fluxes, and thus can be used to infer past variations of Earth surface deformation processes, or climate change. Accurate estimates of sediment fluxes over various times scale and space-scale are therefore crucial. Traditionally, offshore sediment sequence volumes take into account only siliciclastic accumulation, the carbonate fraction being considered only as in-situ production (i.e. CaCO3). Here we propose a new geochemical methodology to decipher and quantify the amount of detrital carbonates in comparison to in-situ produced biogenic carbonates. This isotopic approach enables taking into account the export of detrital carbonates, and investigating its effect on sediment budgets. This study, located in the Gulf of Lion, is based on a 300 m long sediment borehole located near the shelf break and covering the last 500 000 years (i.e. 5 glacial-interglacial periods). 86Sr/87Sr isotopic data (0.70809 to 0.70858) are significantly less radiogenic than modern seawater (i.e. 0.7092) and show fluctuations in agreement with stratigraphic and climatic variations. These results suggest an unsuspected high export of detrital carbonates from the catchment area during both glacial (between 55 to 85% of the sedimentary carbonate fraction) and interglacial (between 30 to 50%) conditions. Thus, not only do detrital carbonate fluxes need to be factored into sediment flux calculations, these results suggest that detrital carbonate components could potentially have a strong influence on carbonate 86Sr/87Sr ratios when not obtained from microdrilled biogenic carbonates, such as the entirety of the Precambrian Sr chemostratigraphic record.

被动大陆边缘（passive margin）的陆源沉积收支记录了过去的沉积通量变化，因此可用于反演地球表面变形过程或气候变化的演化历史。准确估算不同时间与空间尺度下的沉积通量，因此至关重要。传统上，近海沉积序列的体积仅考虑硅质碎屑堆积（siliciclastic accumulation），碳酸盐组分仅被视作原位生成（in-situ production，即碳酸钙CaCO3）。本研究提出一种全新的地球化学方法，用以解译并量化碎屑碳酸盐（detrital carbonates）与原位生成生物碳酸盐（biogenic carbonates）的相对含量。该同位素方法可实现对碎屑碳酸盐搬运过程的考量，并探究其对沉积收支的影响。本研究聚焦于里昂湾（Gulf of Lion），基于一处位于陆架坡折附近的300米长沉积钻孔，该钻孔的沉积记录覆盖了过去50万年，即5个冰期-间冰期旋回。本次测得的86Sr/87Sr同位素比值范围为0.70809至0.70858，其放射性显著低于现代海水（0.7092），且波动特征与地层学及气候变化相一致。研究结果显示，无论是冰期（沉积碳酸盐组分中碎屑碳酸盐占比55%~85%）还是间冰期（30%~50%），流域（catchment area）来源的碎屑碳酸盐搬运量均远超此前预期。由此可见，不仅沉积通量计算需要纳入碎屑碳酸盐通量的考量，本研究结果还表明：若未通过微钻探取生物碳酸盐样品，例如针对全部前寒武纪锶化学地层（chemostratigraphic）记录的分析场景，碎屑碳酸盐组分可能会对碳酸盐的86Sr/87Sr比值产生显著影响。