Fe speciation and Fe/Al ratio in the sediments of southeastern Arabian Sea as an indicator of climate change

Aluminium, titanium and iron are the major lithogeneous contributors to the marine sediment and their abundance varies strikingly with the intensity of monsoonal precipitation. Al and Ti concentrations in a sediment core (SK-129/GC-05) from southeastern Arabian Sea exhibit a very strong positive correlation (r = 0.92, n = 140) suggesting their lithogenous association, whereas Ti and Fe does not correlate so strongly (r = 0.44). This suggests that part of Fe is structurally unsupported, as is evident by the two-fold increased Fe/Al ratio (0.87 average) compared to upper continental crust (0.44). The presence of structurally unsupported Fe is confirmed by the sequential leaching of Fe which is associated with different fractions such as exchangeable (0.98%), carbonate (1.98%), Fe–Mn oxide (30.8%), organic carbon (4.7%) and biogenic opal (11.4%) which together constitutes nearly 50% of the bulk Fe content. There is a good correlation between time series Fe/Al and differential solar insolation, suggesting warmer and humid climatic conditions resulted in intense weathering of hinterland rocks, associated with increased monsoonal precipitation, which probably decoupled soluble Fe and transported it to the southeastern Arabian Sea.