ISOTOPIC MEASUREMENTS BY LASER-ABLATION-ICP/MS IN CORALS, A NEW LOOK AT AN OLD PROBLEM

Many studies have established the advantages of analyzing a suite of tracers from the same coral skeleton, allowing simultaneous determination and decoupling of a range of environmental variables. One microbeam technique that is becoming widely used for geological samples is laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). Laser ablation micro sampling requires minimum sample preparation and allows rapid analysis of solid-state samples at resolutions below 20 microns and has the potential to simultaneously analyze up to 20 trace elements. In addition, because the instrument is a mass spectrograph, it also determine isotopic ratios. In this study, isotopic ratios of boron, strontium and lead were measured to assess the history of environmental change. For example, boron isotopes measure changes in pH of the precipitating solutions, strontium isotopic ratios provide insight into sources of waters and the temperature record in the corals, and lead isotopic ratios are used to discriminate between anthropogenic and natural sources, and oceanic and continental sources. Using LA-ICP/MS, the lead isotopic composition of corals in the Florida Keys and on the west Florida shelf showed a dramatic increase between the years 1940 and 1945. This shift coincides with increase of warfare activity during which a significant amounts of oil was added to the reef system by the sinking of ships. There is also a strong relationship between the lead isotopic distribution and increased aluminum concentration in the corals and the occurrence of fresh water pulses transported from the Mississippi River. The boron isotopic record over the last two decades reflects the changes in climate. The data revels a strong relation in the distribution of these isotopes with shifts in the El Nino and North Atlantic Oscillation (NOA) cycles. Although the relationship is not as clear as lead and boron, there is variation in the strontium ratios, particularly from sites near shore. These variations cast doubt on the reliability of using strontium as the thermometer in these locations.