RECONSTRUCTING ANNUAL SEAWATER TEMPERATURE CYCLES USING STABLE ISOTOPE PROFILES IN MODERN BRYOZOANS FROM THE SNARES PLATFORM, NEW ZEALAND

To better predict future climate change, one must quantify and understand past climate variability. The further back in time we go, the more we must rely on proxies. The morphology and geochemistry of bryozoan carbonate skeletons have been used as effective marine proxies. The bryozoan Melicerita chathamensis produces colonies of single blade-shaped branches that exhibit visible growth checks where the branch width constricts. If these are annual, then the colonies can be aged and paleoseasonality as well as inter-annual variation can be quantified. The purpose of our research was to test the use of stable isotope profiling in this species as a method for determining if the growth checks represent annual temperature cycles.

On the Snares Platform south of New Zealand at 168 m water depth, six living colonies of the bryozoan M. chathamensis were collected on 7 Dec 2011. We applied three independent methods to determine colony age. First, each colony was X-rayed to visually determine the location of the growth checks. Second, branch width was measured for each zooid generation along the growth axis. Third, for C and O stable isotope analysis, we micromilled 160 skeletal carbonate powder samples parallel to the growth tip down the colony axes. From these we created isotope profiles along the colony axes.

The colonies measured 9.3-29.5 mm tall (mean: 20.6 mm). δ¹³C values ranged from 1.1 to 2.6‰VPDB (mean: 1.7‰VPDB), and δ¹⁸O values corrected for MgCO₃ content ranged from 0.9 to 2.1‰VPDB (mean: 1.4‰VPDB). δ¹⁸O-derived temperatures had a range of 4.0°C and a mean of 11.6°C with the mean of the coldest 10 measurements of 9.4°C and the warmest 13.4°C. X-ray images suggested colony ages of 1-7 yr (mean: 4.0 yr). Branch widths suggested colony ages of 2-5 yr (mean: 3.7 yr). δ¹⁸O values suggested colony ages of 0.5-13 yr (mean: 5.0 yr).

As δ¹⁸O values from the six growing tips yield a mean temperature of 10.8°C, identical to the measured value of 10.8°C on the sea floor during sampling, we conclude that the method works. The growth checks correlate with the cooler temperatures and are annual with a possible additional higher frequency algal bloom signal. Ongoing morphologic study of these same colonies (see Hageman et al., this volume) will shed light on the simultaneous zooecium-level morphologic response to the environmental changes.