Trace Elements in Deep-Sea Black (Antipatharian) Coral Skeletons

The proteinaceous skeletons of deep-sea Antipatharian 'black' corals are a new proxy archive that has shown promise for providing high resolution marine records in a wide range of waters. Although stable isotopes have been used successfully for palaeoceanographic reconstruction, there have been few studies of trace metals in these skeletons. In this thesis we study a suite of trace elements in black coral skeletons to assess their utility in paleoenvironmental reconstructions.

Fifty black corals were sampled from the NIWA Invertebrate Collection, broadly distributed around New Zealand from ~25˚S to 47 ˚S and 165˚E to 155˚E. Small powder samples were taken from the outer layers of the coral skeleton, dissolved in nitric acid and analysed by ICP-MS using a new methodology developed as part of this thesis. We critically evaluate this new method, quantifying the limits on accuracy and intrinsic reproducibility, and comparing this to the range of concentrations found in coral specimens.

A necessary step in evaluating the potential for trace elements to provide palaeoenvironmental information is to understand how trace elements become incorporated into the corals’ skeletons. Questions include whether uptake is passive or active (i.e. biologically mediated); and whether trace elements derive from the coral’s food (originating in the surface ocean) or from the ambient water in which the coral is growing.

In this thesis we explore these questions by studying spatial patterns of coral trace elements to determine if they show similarity to surface or intermediate-depth ocean trace element distributions. We investigate the influence of several variables on coral trace elements including proximity to the NZ mainland, the depth at which the corals grew, regional oceanography, and primary productivity. We examine the enrichment of skeletal trace elements compared with both coral tissue and particulate organic material (POM). Finally, we examine subsets of the coral samples that control for several environmental variables in an attempt to isolate the effect of coral size and taxonomy on trace element concentrations. Replicate samples from the same specimens and from different specimens at the same site (or within a 25Km radius) were used to assess differences in trace element content within and between coral specimens, respectively.

Our results indicate that black corals strongly enrich many trace elements (TE) in their skeletons and tissues (Br>Zn>Cd>Mo>V>U>Fe>Cu>Ni>I) at levels up to 10⁸ times higher than seawater concentrations. A large intrinsic variability in TE content was found between and within all black coral specimens. This variability largely obscures any relationship that may indicate a water column origin and mechanism of uptake. Coral taxonomy at a genus level strongly influences most TE concentrations, contributing up to 80% of the variation in some elements. Although largely inconclusive, the data hints at a combination of active and passive uptake pathways from ambient or surface seawater sources for some elements which might warrant further investigation.

In order to advance this field of study further, we suggest that a better understanding is needed of the taxonomic control over trace element incorporation, including biomineralisation and biological utilisation of trace elements by black corals. We also note that there is a lack of data on trace element biogeochemical cycles in the waters around NZ, which would be important in order to better constrain the behaviour of the TEs in black corals and to better evaluate their paleoenvironmental utility.