Environmental Records from Corals and Coralline Sponges

Abstract

This research centers on the extraction of environmental information stored in the calcium carbonate skeletons of corals (Porites) and coralline sponges (Astrosclera willeyana). Elemental variations are measured in these samples using laser ablation inductively coupled plasma mass spectrometry. During this project, techniques were refined and developed to quantitatively measure major (B, Mg, Sr, Ba, U) and minor (Mn, Zn, Pb, REE’s) elemental abundances in corals and coralline sponges.

This method was used to extract seasonal records from a high latitude coral living at its limits of cold-water tolerance in Japan. Seasonal cycles of major elements (B, Mg, Sr, U) were calibrated against in situ instrumental temperature monitors. Calibrations were in good agreement with other published reports. This coral also recorded wind-induced upwelling (both annual and El Niño related) by increases in the Ba/Ca ratio. The trace element annual patterns and slow growth rate also indicated that this coral had very slow/no extension during the cold winter months when water temperature was below 18 oC.

Cores from Porites sp. coral colonies were also collected from inshore, mid-shelf and outer reef localities (central Great Barrier Reef) to test the robustness of the major elemental SST and runoff proxies (B/Ca, Mg/Ca, Sr/Ca, Ba/Ca, U/Ca). The inshore reefs selected for this study are heavily influenced by river runoff whereas the mid-reef and outer-reef locations are not. It is shown that Ba/Ca provides an excellent proxy for river runoff. Time series analyses of Sr/Ca, U/Ca, B/Ca and Mg/Ca are compared to in situ sea surface temperature (SST) and/or IGOSS NMC weekly satellite SST to provide calibrations for these elements. Previous workers have noted differences in the calibration of Sr/Ca vs. SST; this LA-ICP-MS dataset shows a slight variation between different corals. This suggests small-scale intra- coral variability. Both the U/Ca and Mg/Ca have calibrations within error for mid-shelf and outer reef corals but the calibrations differ for the inshore corals. Sr/Ca and B/Ca appear to be the most robust of the elemental temperature proxies in terms of recording water temperature. Measurements of coral manganese suggest a seasonal cycle closely linked to solar radiation and wind. Increased solar radiation may increase the dissolved seawater Mn that result in corals having elevated concentrations of Mn during the summer.

The use of corals as recorders of marine pollution was examined on the island of Misima in Papua New Guinea where open-cut gold mining commenced in 1989. This mining caused increased sedimentation affecting the nearby fringing coral reef to varying degrees, causing coral mortality (complete suffocation) in some areas. This sediment is made up of completely weathered quartz feldspar, greenstone and schist. These rocks have distinct chemical constituents (rare earth elements [REE], zinc and lead etc.), which are entering the near-shore environment in considerably higher than normal concentrations. Eight coral colonies (2 from high sedimentation, 2 transitional, 2 minor and 2 unaffected control sites) were analyzed for Mn, Y, La, Ce, Zn and Pb. All sites show low steady “background” levels prior to the commencement of mining. After mine construction began in 1988, all sites aside from the control show dramatic increases of Mn, Y, La, and Ce associated with the increased sedimentation. Zn and Pb increase after 1989 when ore processing began. The concentration of these elements in these corals decreases as the distance from the mine increases. Rare earth elements (REE) measured in two corals suggest a pattern different from “normal” seawater. When the coral REE pattern is compared to seawater an enrichment of the light and middle REE’s appear. The heavy REE’s are depleted relative to the seawater pattern. This suggests the nearshore seawater REE’s are influenced by island weathering.

Coralline sponges have been proposed as a new source of tropical paleoclimatic information. Profiles of d 13 C in coralline sponges have documented (better and more accurately than corals) the atmospheric increase of 12 C associated with increased fossil fuel consumption. Due to their very slow growth rates ~0.2 mm yr -1 sponges are better suited to recording and providing long-term environmental information rather than annual information. These sponges appear to smooth the record stored in their skeleton by adding secondary aragonite near the base of the living tissue layer. This smoothing limits their use as annual environmental recorders but still enables their use for decadal or longer environmental fluctuations. Smoothed records of Sr/Ca from five sponges around the Southwest Pacific suggest that these sponges are able to capture 5 yr. and longer seawater temperature anomalies over the past 50-200 years. The temperature sensitivity of Sr/Ca in sponges is 7 to 12 times larger than corals with ~0.7-0.9 mmol/mol DSr/Ca per oC for sponges compared to ~0.07 mmol/mol DSr/Ca per oC for corals.