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Objectives and work progress

The primary objective of this proposal is to test the utility of planktonic foraminiferal boron-to-calcium (B/Ca) ratios as a proxy for seawater pH using samples collected from Time Series sediment traps.

Our work over the past year has focused on the following issues:

1. Developing new methodology for measuring B/Ca by ICP-MS

We have modified our routine method for trace-metal analysis for measuring also B/Ca ratios.  The main modification allows us to add ammonia gas directly into the spray chamber (together with argon), which greatly reduces the boron memory effect during the ICPMS analysis. In addition we now purify our ddH₂O and reagents for B. These modifications allow us to obtain low-level B measurements with sufficient precision and accuracy for all the other elemental ratios.  However, we have found that this method is not sensitive enough for very small samples, which may limit our ability to measure B/Ca on foraminifera during non-bloom periods (see below). 

To handle small samples we have developed a new method based on the micro-FAST introduction system for measuring B/Ca in small volume samples. In the regular method, 20-30 shells averaging about 200μg are prepared in a final volume of 400μL for solution analysis. The use of sediment trap material can be limited by sample size with only several individual foraminifera available during low abundance periods (e.g., December-April). The analysis of 1-5 shells (5-25 μg) requires the determination of B/Ca at a much lower concentration. The development of the micro-FAST flow injection sample introduction system coupled with high-resolution ICP-MS allows for the analysis of sub 100μL volumes Single foraminifera analysis using microFAST-HR-ICP-MS improves our ability to resolve a seasonal signal and construct a complete calibration (Babila et al., 2009).

2. Sampling of sediment traps material

During the first year we focused on samples from two stations namely i) Bermuda and ii) South China Time Series. 

i) To date graduate student Tali Babila has completed the sampling of 2 full annual cycles from 1994-1995 in two weeks resolution.  We sampled four planktonic foraminifera (G. ruber, G. sacculifer, P. obliquiloculata and N. dutertrei).  Our initial analysis has indicated that B/Ca varies with shell size.  Consequently, we have divided each sample into discrete size fractions that will be analyzed separately.  For each size fraction are weighing the foraminifera shells to test the hypothesis that changes in surface water calcite saturation are expressed in individual size-normalized shell weight.  Ultimately we would apply the same method to samples from the early 80's and recent ones to assess whether the recent acidification of ocean surface water over the past several decades has affected the calcification of planktonic foraminifera.  In addition, graduate student Tali Babila participated on a 4-day cruise to recover the sediment trap at the Bermuda station.  These samples will be used as our most recent calibration period and will compare with samples from the 1980s.

ii) Graduate student Kuo-Fang Huang has completed the analysis of samples from the South China Sea traps.  These samples cover 5 months interval in 2004 (G. ruber ) and will be compared with the data from Bermuda.  These results are also compared with data obtained from the underlying surface sediments to assess the compatibility between fresh and fossil material.  

3.  Surface sediment calibration

To test the reliability of B/Ca in fossil material we have also measured B/Ca in core-tops planktonic foraminifera from different ocean basins.  Results obtained on core-tops samples will be compared with data obtained from the sediment traps.

The data collected so far already shed new light on the geochemical controls on B/Ca in planktonic foraminifera.  Previous studies have debated whether temperature plays a substantial role in the incorporation of B into the calcitic shells with opposing results from these studies.  Our data (see figure 1) clearly demonstrate that overall there is a strong temperature influence on B incorporation although within each region there might be additional, hitherto not well understood, local effects (in addition to pH).  In contrast to the previous studies that used a limited temperature range for their calibration, we are using a wide range and hence, our results are less affected by local effects.  These results are now in preparation for publication.

Personnel and budget

While several students have been involved in this project, it is becoming the thesis subject of PhD candidate Tali Babila.  Tali passed her qualifying exams last summer and since then has turned her full attention to the project.   Taking classes and studying for the qualifying exams has slowed her down in the past year.  Given this, I have used matching funds to support her last year with the idea that funds from this grant will support her once she is 100% on the project.   However, due to administrative error she has been kept on matching funds to this day.   Give her current progress and the budget status I intend to ask for no cost extension for the third year, which will allow her to finish the project and her thesis.   In addition I would like to get a post doc involved in the project so we can make significant progress by the end of next year.  

Abstracts and manuscripts in preparation partially supported by this grant

Babila, T. (2009). Development of B/Ca in planktonic foraminifera as a proxy for seawater pH.  Poster Urbino Summer School of Paleoeanography, Italy

Babila T., Field MP., and Rosenthal Y. (2009). Determination of B/Ca in planktonic foraminifera as a proxy for  seawater paleo-pH. 8^th International Sector Field ICPMS Conference in Ghent, Belgium.

Huang, K-F., Rosenthal, Y. et al., (in preparation).  Reconstruction of change in surface water pH and pCO₂ in tropical  South China Sea using foraminiferal multi-proxies approach.

Figure 1:  (Provisional data) Sediment trap and core top samples from South China Sea (this study) compared with published results.  The new data suggest a strong temperature control on B/Ca in the surface water species G. ruber.  This relationship can be used to evaluate the additional pH dependence.