C. Pelejero, E. Calvo, S. Eggins and M.T. McCulloch
Since the beginning of the Industrial Revolution,
burning of fossil fuels has increased the atmospheric CO2 content
from ~280 to almost 370 ppmv, a level unprecedented in the last
420,000 years. This excess atmospheric CO2 is largely absorbed
by the oceans, which are one of the main carbon reservoirs of
our planet, basically due to the high solubility of CO2 in seawater
and biological processes that transfer carbon from surface into
deep waters. However, the capacity of the oceans to respond to
this large amount of anthropogenic CO2 is still not known.
A multi-proxy approach to evaluate the role
of the oceans in absorbing CO2 is being carried out by means
of molecular biomarkers and isotope studies of marine sediments
south of Australia, namely C37 long chain alkenones and boron
isotopes ((11B). Alkenones are compounds specifically synthesized
by phytoplanktonic Haptophyta algae and are characteristic biomarkers
in sediments from all oceans. Their relative abundances, expressed
as |
the UK37 index, show a close relationship
with the temperature of the waters where these compounds were
biosynthesized, forming the basis of a well established paleothermometer
for marine waters (Figure 1). So far, the UK37 method has been
set up at Geoscience Australia where a recently acquired Dionex
Accelerated Solvent Extraction device (ASE 2000) has been optimised
for this purpose, allowing a more rapid sample extraction, increased
automation and lower solvent consumption and exposure. Apart
from estimating paleo-sea surface temperatures (SST), these compounds
are also taken as qualitative indicators of paleo-marine productivity
as well as paleo-pCO2 (through the analysis of the carbon isotopic
composition, (13C, of these alkenones).
On the other hand, the response of seawater
pH to past changes in atmospheric CO2 will also be addressed
by means of the analysis of boron isotopes on foraminifera and
corals, given that seawater B isotopic composition changes with
pH. Data on (11B will be obtained by means of multiple collector
inductively coupled plasma mass spectrometry (MC-ICP-MS). In
this sense, the new Finnigan Neptune MC-ICP-MS set up at RSES,
which provides near constant mass fractionation and high precision,
is being optimized to obtain a robust method to routinely analyse
boron isotopes. |