Sedimentary phosphorus and iron cycling in and below the oxygen minimum zone of the northern Arabian Sea

Abstract

Abstract. In this study, we investigate phosphorus (P) and iron (Fe) cycling in sediments along a depth transect from within to well below the oxygen minimum zone (OMZ) in the northern Arabian Sea (Murray Ridge). Pore-water and solid-phase analyses show that authigenic formation of calcium phosphate minerals (Ca-P) is largely restricted to where the OMZ intersects the seafloor topography, likely due to higher depositional fluxes of reactive P. Nonetheless, increased ratios of organic carbon to organic P (C org /P org ) and to total reactive P (C org /P reactive ) in surface sediments indicate that the overall burial efficiency of P relative to C org decreases under the low bottom water oxygen concentrations (BWO) in the OMZ. The relatively constant Fe/Al ratio in surface sediments along the depth transect suggest that corresponding changes in Fe burial are limited. Sedimentary pyrite contents are low throughout the ∼25 cm sediment cores at most stations, as commonly observed in the Arabian Sea OMZ. However, pyrite is an important sink for reactive Fe at one station in the OMZ. A reactive transport model (RTM) was applied to quantitatively investigate P and Fe diagenesis at an intermediate station at the lower boundary of the OMZ (bottom water O 2 : ∼ 14 µmol L −1 ). The RTM results contrast with earlier findings in showing that Fe redox cycling can control authigenic apatite formation and P burial in Arabian Sea sediment. In addition, results suggest that a large fraction of the sedimentary Ca-P is not authigenic, but is instead deposited from the water column and buried. Dust is likely a major source of this Ca-P. Inclusion of the unreactive Ca-P pool in the C org /P ratio leads to an overestimation of the burial efficiency of reactive P relative to C org along the depth transect. Moreover, the unreactive Ca-P accounts for ∼85 % of total Ca-P burial. In general, our results reveal large differences in P and Fe chemistry between stations in the OMZ, indicating dynamic sedimentary conditions under these oxygen-depleted waters.

Intact polar and core glycerol dibiphytanyl glycerol tetraether lipids in the Arabian Sea oxygen minimum zone. Part II: Selective preservation and degradation in sediments and consequences for the TEX86

“…and increased with increasing bottom water oxygenation. Pore water profiles, 210 Pb dating and Ti/Ca ratios of sediments from the same locations showed that sedimentation was relatively continuous with no evidence for winnowing or gravity flows (Kraal et al, 2012).…”

Carbon and Phosphorus Cycling in Arabian Sea Sediments across the Oxygen Minimum Zone

“…This rate of CFA growth is high compared to high-productivity deep-sea settings (34), but is within the range of high-productivity continental margin settings [4,7]. These downcore records of CFA P might also be influenced to some degree by variation in the input of dust through time; dust in this region contains some CFA [22], which would presumably be delivered to the seafloor with little or no geochemical alteration before burial [2].…”

“…In particular, earlier scenarios called for the vast majority of reactive P sedimentation to the seafloor to be in the form of organically-bound P compounds. A variety of studies focusing on low-oxygen environments note somewhat higher-than-Redfield ratios in the organic matter fraction [19], but also find a significant role of iron in the burial and diagenetic cycle of P [20][21][22][23]. This dialog, and a satisfactory understanding of coupled C and P cycling in modern environments, can be informed by continued detailed studies focused on environments that receive similar sedimentary inputs but span a range of oxygen contents to clarify the role of oxygenation.…”

“…This redistribution (so-called 'sink-switching'; Ruttenberg and Berner [16]) may greatly affect the sedimentary record of P cycling. An additional source of mineralized P delivery to the seafloor is in fish teeth, bone, and scale material, which globally comprises ~30% of total P accumulation [7], and aeolian/riverine CFA and diatom-bound P P [22]. In the anoxic Saanich Inlet, British Columbia, the C org :P org ratio increased from about 160 in surficial sediments to above 800 at depth (60 m or 7,000 ybp; Filippelli [5]).…”

“…Our approach is to examine C and P geochemistry in surficial samples from a wide range of site water depths, and couple this array with four short cores selected across the OMZ and aimed at understanding diagenesis of these elements upon shallow burial. Our study is supplemented by several other examinations of P geochemistry in the region, including an analysis of the role that phosphogenesis (the authigenic formation of CFA) plays in the ultimate burial of P [7,22,24] and the interplay between paleoredox variations and the reflux of P back to the water column, perhaps fueling higher productivity in the Oman and Somalian coastal upwelling zones and indeed across the entire basin. This link between paleoredox and surface productivity has also been the focus of other regional-scale studies in the Mediterranean, where the redox-P-productivity process was a significant factor driving Milankovitch-scale ecosystem changes during the formation of Mediterranean sapropel-marl sequences during the latest Miocene [25,26].…”