Quantifying carbonate dissolution and calibration of foraminiferal dissolution indices in the Somali Basin

Abstract

Two sediment traps moored off Somalia in 1992–1993 collected similar settling fluxes of carbonate and siliceous shells formed by various plankton groups. Planktic foraminifera showed large seasonal variations, with more than 74% of the total planktic foraminifera flux collected during the SW monsoon (summer upwelling), when Globigerina bulloides was dominating along with Globigerinita glutinata and Neogloboquadrina dutertrei. The intermonsoon and NE monsoon assemblages were dominated by Globigerinoides ruber. We used the trap records as our ‘no dissolution’ reference for comparison with three boxcore recorders in order to quantify the carbonate dissolution along a depth transect. Dissolution increases downslope, from Station 905 to 907 and 915 at depths of 1567 m, 2807 m and 4035 m, respectively. The carbonate fraction of the sediment at Station 915, which is located near the CCD, is the most affected by dissolution, with more than 97% of the planktic foraminifera dissolved. Here, the planktic foraminifera assemblage is strongly modified, with thick walled species such as N. dutertrei, Globorotalia tumida and Pulleniatina obliquiloculata as the most resistant. It is not representative of the settling assemblage. The planktic foraminiferal assemblages of the sediment surface at Stations 905 and 907 remain similar to the trap assemblages and the foraminifera are well preserved, although only 25% of foraminifera are apparently preserved at Station 905 and 8% at Station 907. Those numbers are surprisingly low and infer that only a small fraction of the foraminiferal carbonate production is buried and removed from the carbon cycle. This discrepancy between the export and buried flux is partly be due to (bio)mechanical destruction by benthic processes and to supralysoclinal dissolution, due to metabolic CO 2 generated by the benthic organisms. Another important factor is the interannual variability of the productivity, well known in the Arabian Sea. The calibration of commonly used foraminiferal dissolution indices (percentage of foraminiferal fragments, percentage of resistant species, foraminiferal dissolution index (FDX)) to our data only shows reliable results for high dissolution levels (>97%). Off Somalia the most accurate of the proxies is the percentage of foraminiferal fragments compared to the other methods tested, i.e., FDX, planktic foraminiferal loss (L), percentage of radiolarians and diatoms, percentage of benthic foraminifera. The species assemblage appears to be not significantly modified by dissolution unless the estimated shell loss is high, >92% of the arriving shells in our samples. This level is expressed in the percentage of fragmentation, the percentage of radiolarians and diatoms, the percentage of resistant foraminifera species, and FDX as 80%, 35%, 25% and 1.8, respectively. The relative abundance of Globigerina bulloides is a valid SW monsoon/upwelling proxy only when dissolution is moderate (<92%). Globigerina bulloides and Globigerinoides ruber have similar burial efficiencies and susceptibilities to dissolution. Thus, the ratio G. bulloides/ G. ruber is a valid proxy for past changes in the intensity of the SW monsoon even in the strongly dissolved samples. In our sediment record the ratio G. bulloides/ G. ruber indicates that the SW monsoon was stronger in the recent past than in 1992–93.