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.
