Abstract
This study demonstrates that intracrystalline organic matter in coral skeletons is well preserved over century timescales. The extent of preservation of organic matter in coral skeletons was investigated by measuring total organic carbon (TOC), total hydrolyzable amino acid (THAA), chloropigment, and lipid concentrations in 0–300 year old annual growth bands from Montastraea annularis (Florida Keys) and Porites lutea (Red Sea). Organic matter intrinsic to the calcium carbonate mineral (intracrystalline) was analyzed separately from total skeletal organic matter. The Red Sea coral had less TOC (0.02–0.04 wt%) than the Florida Keys coral (0.04–0.11 wt%), but a higher percent of intracrystalline organic matter in all annual bands measured. Carbon in the form of THAA, most likely from mineral-precipitating proteins, contributed 30–45% of the TOC in both corals. Carbon in lipids represented about 3% of the TOC in the coral skeletons. Chlorophyll-a and b were present in annual bands where endolithic algae were present, but these compounds were minor contributors to TOC. The distribution of specific organic compounds showed that organic matter was well preserved throughout the time period sampled in both the total and intracrystalline pools. Variations in THAA were not correlated with TOC over time, suggesting that organic matter that is involved in biomineralization, like amino acids, may be deposited in response to different environmental factors than are other components of skeletal organic matter. Differences in the quantity and composition of organic matter between the two corals investigated here were assessed using principal components analysis and suggest that location, species and skeletal structure may all influence organic matter content and possibly the degree of physical protection of organic matter by the coral skeleton. Further, our study suggests that intracrystalline organic matter may be better protected from diagenesis than non-intracrystalline organic matter and may therefore be a more reliable source of organic matter for paleoceanographic studies than total skeletal organic matter.
Highlights
Investigations of the composition and stability of organic matter in biominerals suggest that carbonates and silicates can be important conveyers of relatively unaltered organic matter in the marine environment (e.g., Carter and Mitterer, 1978; Maita et al, 1982; Collins et al, 1992)
Annual bands from 1939 –1982 (TRIII) that had been stored since that earlier study were analyzed for total organic carbon (TOC), insoluble nitrogen, chloropigments and total hydrolyzable amino acids (THAA)
This study describes the composition and concentration of a large fraction of the organic matter in coral skeletons of various ages
Summary
Investigations of the composition and stability of organic matter in biominerals suggest that carbonates and silicates can be important conveyers of relatively unaltered organic matter in the marine environment (e.g., Carter and Mitterer, 1978; Maita et al, 1982; Collins et al, 1992). Inorganic compounds in coral skeletons are presently used as paleoceanographic proxies due to the well-defined stratigraphy of the annual carbonate bands (e.g., Druffel, 1997). We investigate the century-scale preservation of organic matter in the skeletons of two species of mound-forming scleractinian corals with the hope that understanding temporal variability in composition and concentration of coral skeletal organic matter will lead to the development of new paleoceanographic proxies in the future
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