Trace element geochemistry of marine biogenic particulate matter

Abstract

Plankton samples have been carefully collected from a variety of marine environments under the rigorous conditions necessary to prevent contamination for major and trace-chemical analysis. Immediately after collection, the samples were subjected to a series of physical and chemical leaching-decomposition experiments designed to identify the major and trace element composition of particulate carrier phases. Elements examined through some or all of these experiments were: C, N, P, Mg, Ca, Si, Fe, Mn, Ni, Cu, Cd, Al, Ba, and Zn. Emphasis was placed on the identification of trace element/major element ratios in the biogenic materials. The majority of the trace elements in the samples were directly associated with the non-skeletal organic phases of the plankton. These associations included a very labile fraction which was rapidly released into seawater and a more refractory fraction which involved specific metal-organic binding. Calcium carbonate and opal were not significant carriers for any of the trace elements studied. A refractory phase containing Al and Fe in terrigenous ratios was present in all samples, even from the more remote marine locations. The concentration of this carrier phase within the plankton samples varied in proportion to the estimated rate of supply of terrigenous matter and in opposition to the rate of production of the biogenic particulate matter. The aluminosilicates contributed insignificant amounts to the other trace elements studied. A trace concentration of particulate Al was identified which was more labile and associated with the organic fractions of the samples. Variations in the surface water concentrations of dissolved Cu, Ni, Cd, and Zn with respect to P are compared to the ratios measured in the plankton samples and their regeneration products. The trace element/major element ratios ,in the residual plankton materials can be combined with estimates of the carrier fluxes to account for the transport of trace elements required to maintain their deep enrichment. A variety of processes determining the geochemical cycles of specific trace elements were identified. As much as 50% of the Cd, Ni, Mn, and P are rapidly released from plankton and recycled within the surface ocean. During this process, the metal/P ratio in the residual particles must decrease by 10-30% for Cd and increase by a factor of 2-4 for Ni and Cu to balance their deep enrichments. Although Mn is taken up and regenerated by plankton, the magnitude of this process is small with respect to other non-biogenic Mn fluxes and has very little influence on its dissolved distribution. The Ba content of all known surface carriers is insufficient to account for the deep enrichment of Ba. A secondary concentration process results in the formation of significant particulate Ba within the upper thermocline.