Source-to-sink micronutrient cycling between seawater, sediment, and nodules in the South Pacific.

Abstract:

Primary productivity in the Southern Ocean is limited or co-limited by the availability of Fe and Co in seawater. To understand the origin and evolution of these highly productive marine ecosystems, the history of metal micronutrients cycling must be investigated. In the South Pacific, water column data from the GEOTRACES GP16 transect demonstrates that the continent, oxygen deficient zone, hydrothermal plume, and dust uniquely affect the distribution of many metals in seawater. Underlying the GP16 transect west of 110°W, there are oxic pelagic sediment and ferromanganese nodules, which are terminal depocenters for many metals in seawater. Here, we examine the patterns of multiple elements (Mn, Fe, Co, Ni, Cu, Zn) in oxic marine sediment and hydrogenetic nodules to investigate how heterogenous metal distributions in seawater are mapped onto the seafloor.

For comparison with the GEOTRACES GP16 water column data, we compiled geochemical data from 10 core sites (402 bulk sediment samples) and 51 hydrogenetic nodules located between 7°S-20°S and 80°W-160°W in the South Pacific Ocean. Ratio plots, ternary diagrams, and spatial maps compare the seawater, sediment, and nodule compositions. Results reveal element patterns with water depth and distance from the East Pacific Rise (EPR). For example, west of the EPR (115°W-135°W), sedimentary Mn/Fe decreases from ~0.5 to ~0.2 mol/mol with increasing distance from the ridge, whereas nodule Mn/Fe exhibits no systematic relationship (ranging from ~0.6-2 mol/mol). The seawater data reveal a pattern opposite to that in bulk sediment, with bottom water dissolved Mn/Fe increasing with distance west of the EPR, albeit over a smaller range (~0.2-0.3 mol/mol). We present hypotheses to explain the trends of Mn/Fe in seawater, sediment, and nodules and also discuss other elements including Co, Ni, Cu, and Zn. This exercise contributes toward constructing a comprehensive framework for understanding how metal proportions and distributions in sediment and nodules can be used as a proxy for past micronutrient cycling in micronutrient-limited regions of the ocean.