OLI Grant: Development of Ion Microprobe Technology for Microscale Measurement of Otolith d180


Grant Funded: 2002

Proposed Research

We propose to develop Ion Microprobe technology, widely used to study the chemistry of the inner earth, for application to problems in Ocean Biology. The WHOI Cameca 1270 Ion Microprobe is the only instrument capable of measuring microscale variations in the oxygen isotope ratio (d18O) of biological carbonates with a precision that enables it to be used as a tool to trace the origins, migrations and life history patterns of marine organisms. d18O is one of the most important and definitive isotope tracers; the abundance of 18O relative to 16O fish otoliths, coral skeleton, mollusc shells and even whale baleen, has been used to address questions in fisheries biology, conservation and climate change. The utility of the technique however, is limited by the amount of sample required to make the measurement. Conventional stable isotope ratio analysis (SIRA) requires bulk sampling and large sample sizes that preclude detection of fine scale variability. It is this fine scale variability that is key to identifying the natal origins of larval fishes, locating spawning grounds, and tracking the daily and seasonal migrations of our economically important fish stocks.

The WHOI Cameca IMS 1270 ion microprobe is a remarkable instrument that employs a primary beam of Cesium ions, several microns in diameter, to measure the chemical composition of carbonate skeletons in-situ, by a process called sputtering. This enables high-resolution analysis of elemental and isotope ratios of select portions of a sample that are too small to be seen with the naked eye. In this project, we will determine the exact timing of larval and adult migrations between coastal and open ocean waters for two important fish species: Atlantic croaker (Micropogonias undulatus) and the American Shad (Alosa sapidissima). We will develop ion microprobe capabilities to construct d18O profiles across the otoliths at sub-weekly resolution, extending from the micron-sized larval core through the adult growth bands. Variations in otolith d18O of up to 8 ?, well within the measurement precision of 0.5 ?, are expected as fish move to and from the brackish estuarine environments to the open coast in a cycle of spawning and migration. The presence of daily growth bands in the otolith structure will enable us to correlate isotopic excursions with the life history of each fish. This will be the first microbeam analysis of fish otolith d18O. By circumventing the limitations of conventional analysis, our approach will find widespread application in many areas of Ocean Biology.