Mixing depths and sediment accumulation rates on an arid tropical shelf based on fine-fraction Pb-210 analysis

摘要

Continental shelves in arid tropical settings present particular challenges to Pb-210-based analysis of sedimentation rates and surface mixing owing to the combination of coarse sediment, deep and year-round bioturbation, and lower atmospheric flux of Pb-210. The modern continental shelf at the northern terminus of the hyper-arid Gulf of Eilat/Aqaba (GOE) receives flood-runoff of siliciclastics, producing a mixed seabed with similar to 20% carbonate that is dominated by larger benthic foraminifera and mollusks. Focusing analysis on only the fine-fraction of deep-penetrating cores (>= 70 cm) from 15 to 40 m water depths yields reliable Pb-210 profiles, in contrast to analysis of bulk sediment (graphical abstract). Sedimentation rates increase offshore five-fold, from 0.01-0.04 cm/y in 15 m water depth to 0.21-0.27 cm/y in 30-40 m depths, reflecting offshore redistribution of flood-delivered siliciclastic sediments away from the wadi mouth, a result also supported by an offshore increase in the inventory of excess Pb-210. In contrast, the thickness of the surface mixed layer (SML) decreases from > 30 cm to similar to 20 cm with proximity to anthropogenic stressors (channelized flood runoff, historic release of sewage and operation of fish cages), which we attribute to the suppression of macrobenthic burrowers. The rate of sedimentation on the Gulf shelf -away from dynamic bypassing in the 15 m shoreface - is comparable to other tropical carbonate shelves rich in large benthic foraminifera, and is higher than rates documented on the adjacent slope, increasing confidence in this approach to( 210)Pb analysis using only the fine fraction. Analysis of the fine fraction rather than bulk sediment would be a useful adjustment to Pb-210 methodology in any area with scarce fine-grained sediment.