| Changes in the chemical and biological oceanography accompanying shifts in ENSO conditions in the western tropical Pacific are not well understood and this understanding would be enhanced by high resolution, century-scale proxy records spanning the euphotic zone. Soft corals and black corals are abundant in the western tropical Pacific from the near surface to thousands of meters deep, deposit organic skeleton in concentric bands, and live for hundreds to thousands of years. Geochemical measurements across colony growth axes can serve as proxies for the biogeochemistry of particulate organic matter (POM). Yet, proxy records from these corals in the climatically-sensitive western tropical Pacific have not yet been developed. Here, quantifying the natural variability in organic skeletal delta13C and delta 15N values facilitated comparisons of skeletal records from multiple taxa across a depth transect within the euphotic zone. Similar delta 15N values between black corals and soft corals collected from a reef offshore of Palau suggest these orders feed at the same trophic level while lower a13C values in black corals than soft corals indicate a correction of +1.5‰ is needed to compare delta13C values between orders. In addition, due to chemical alteration of their food, suspended POM, with depth, a +0.25 ‰/10 m correction needs to be applied to a13C values and -0.15 ‰/10 m correction needs to be applied to delta 15N values to compare records from multiple depths. Stable isotopes (delta13C and delta15N) and trace elements (Br, I, Pb, Mn, Cd, Zn, and B) were measured in one Antipathes black coral colony from 5 m and two Muricella soft coral colonies from 85 and 105m, all collected offshore of Palau. Records were dated with a radiocarbon (14C)-derived chronology. The delta 13C records decreased at rates consistent with the oceanic 13C-Suess effect, indicating anthropogenic carbon was a primary control on the delta13C of suspended POM through the top 105 m of the water. Very different delta15N records were derived from the shallow Antipathes colony than the deeper Muricella colonies. Since all three colonies fed on suspended POM in the water column, the dissimilar records indicated different controls on delta 15N values of suspended POM within and below the mixed layer. While changes in the source of POM to the 5 m Antipathes colony with shifts in the relative strength of currents bathing Palau within the mixed layer may drive increases and decreases in shallow delta15 N record, gradually decreasing delta15N values in both of the deeper Muricella records indicated a shoaling of the mean nutricline depth in recent decades. Three radial transects measured by LA-ICP-MS were only reproducible in the Antipathes colony, supporting these corals for trace element reconstructions. This research is the first to develop soft corals and black corals in the western Pacific as proxies of seawater chemistry across the euphotic zone. Together, these corals provide paleoceanographic information on annual to centennial timescale changes in seawater chemistry across the depth range from near surface to thousands of meters deep. |
