The behavior of particulate matter in the Persian Gulf: biogeochemical proxies for source identification

Abstract

The present research study is intended to assess the probable sources of suspended particulate matter (SPM) in the Persian Gulf. SPM was collected at three to four depth resolutions: surface, 10 and/or 25 m, and 2–3 m above the seabed over three research cruises: PGE1901 (May 2019), PGE1902 (November 2019), and PGE2101 (March 2021). The SPM concentration exhibits several maxima along the northern coastline and is higher in winter than spring and autumn. SPM consisted of calcium carbonate (CaCO₃), opaline silica (SiO₂), organic matter, detrital matter from the upper continental crust (UCC), and contaminants derived from industrial activities (e.g., leaking or combustion of fossil fuels). Particulate organic matter (POM) is the major fraction in offshore samples, accounting for up to 84 % of the particulate mass, while the contribution of MnO₂ is lower than 0.1 % during the three seasons. The particulate CaCO₃ content varied more than other major components, being lowest in the northern Strait of Hormuz and up to 40 % in the region near the southern carbonate shelf. Strong coupling among the biochemical parameters Chl-a, bSi, TPP, and TPN was observed during the winter, likely reflecting higher autotrophic contributions to the particulate organic matter (POM). The decrease in TPN from autumn to winter, along with increases in Chl-a and TPP, resulted in three- and two-fold drops in N:Chl-a and N:P ratios, respectively. The concentrations of metal elements decrease in the general order of Ca > Fe > Al > Zn > Ti > Mn > Cr > Cu > Ni > Pb > V > Mo > Cd. Most of the elements exhibit enrichment levels ranging from 50 to 5000 times higher than the values found in the upper continental crust, with notable exceptions for titanium (Ti) and manganese (Mn). Except for cadmium, with a Me/P ratio close to the reported intracellular values, phytoplankton's contribution to the particulate pool of other bioactive trace elements barely exceeds 15 %. The predominant fraction of particulate Pb, Zn, Cu, and Ni is beyond the dust and phytoplankton contribution, possibly from the long-term accumulation of these elements, with some signs of local anthropogenic input.