Supplementary material to “Challenges to Understanding Ocean Circulation During the Last Glacial Maximum”
Published 12 May 2009
Andre Paul and Stefan Mulitza, Center for Marine Environmental Sciences (MARUM) and Department of Geosciences, University of Bremen, Bremen, Germany
Citation:
Paul, A., and S. Mulitza (2009), Challenges to Understanding Ocean Circulation During the Last Glacial Maximum, Eos Trans. AGU, 90(19), 169. [Full Article (pdf)]
MARUM Workshop: Reconstruction of the Glacial Deep Ocean Circulation
Bremen, Germany, 4–7 November 2008
Authors:
- André Paul (corresponding author) MARUM — Center for Marine Environmental Sciences and Department of Geosciences University of Bremen PO Box 33 04 40 28334 Bremen Germany apaul@marum.de
- Stefan Mulitza MARUM — Center for Marine Environmental Sciences and Department of Geosciences University of Bremen PO Box 33 04 40 28334 Bremen Germany smulitza@uni-bremen.de
The climate of the Last Glacial Maximum (LGM, 23–19 kyrs BP) cold period provides the means for evaluating the response of the climate system to large perturbations. It turns out that IPCC-type coupled climate models yield very different results on, for example, Atlantic Ocean meridional overturning rates [Otto-Bliesner et al., 2007]. A 3-day workshop that brought together about 30 international paleoclimatologists (including Thomas Arsouze, Wolfgang Berger, Torsten Bickert, Martin Butzin, Cristiano M. Chiessi, Mea S. Cook, Trond Dokken, Jeanne-Marie Gherardi, Marcus Gutjahr, Ed C. Hathorne, Ben Hickey, Joël Hirschi, Babette Hoogakker, Jörg Lippold, Jean Lynch-Stieglitz, Andreas Mackensen, Olivier Marchal, Thomas Marchitto, Elisabeth Michel, Stefan Mulitza, Ulysses Ninnemann, Kevin Oliver, André Paul, Alex Piotrowski, Matthias Prange, Michael Sarnthein, Luke Skinner and Mara Weinelt) aimed at creating an inventory of existing paleoceanographic records for the LGM (Figure 1) and exploring the potential of individual proxies to serve as paleocirculation indicators and a benchmark for testing climate models.
Benthic δ18O and δ13C measurements are available from about 800 sites with a relatively wide geographical coverage across the world oceans. However, deep-water formation regions and pathways, as well as large areas of the Pacific Ocean, still remain under-represented. Improved calibrations including measurements of water column δ18O and δ13C are required. Inter-laboratory calibrations must be conducted and uniform sample handling, preparation techniques, and reporting standards (including number of shells, size fraction, specimen images, and treatment to remove organic matter, etc.) must be adopted in order to reduce systematic errors.
Benthic Cd/Ca has been reported from about 75 sites in the Atlantic, but additional data are required in the South Atlantic, Southern, Indian and Pacific Oceans. Trace metal/Ca ratios have problems similar to those of stable isotopes, including the needs for better proxy calibration and inter-laboratory calibrations using common standards. The interpretation of both Cd/Ca and Mg/Ca in benthic foraminifera could be significantly improved if bottom water ΔCO32- could be better constrained, for example, through B/Ca measurements. It is recommended that cleaning techniques are clearly described, and that contaminant ratios and average values for reference materials are also reported.
The extraction of seawater-derived neodymium (Nd) isotopes from marine sediments, benthic foraminifera or deep sea corals is a relatively new inorganic proxy with potential to reconstruct water mass sourcing and structure. As yet, worldwide coverage is still sparse, and water mass end-members as well as their variability through time need to be better constrained to improve the use of Nd isotopes to quantify water mass mixing proportions. Furthermore, we need to better understand the chemical mechanisms which add Nd to the ocean, including exchange of Nd with particles (e.g., effects of “boundary exchange”) along continental margins and local input sources such as rivers or continental groundwater discharge which may have modified the signal. The strength in the use of this proxy lies in very different source compositions (e.g. Atlantic versus Pacific), as well as the fact that Nd is not affected by biological productivity.
Reconstructions of Δ14C of deep waters and apparent ventilation ages have the potential to constrain the origin of deep water masses and the rates of ocean circulation in the past. Poorly constrained surface reservoir ages result in questionable chronologies and potentially bias estimates of deep-ocean ventilation. Due to poor spatial coverage, the Δ14C of water mass end-members and the proportion of deep water masses are often not well known. In addition, past changes in the production rate of atmospheric 14C during the LGM and the subsequent deglaciation, which are still poorly understood, lead to an ocean radiocarbon distribution that is not in equilibrium. Nevertheless, ocean circulation models can be tested against the Δ14C data and at the same time provide estimates of 14C reservoir ages in order to narrow down uncertainties in the dating of marine records.
Sediment 231Pa/230Th may also have the potential to estimate the strength of ocean deep circulation owing to the difference in particle reactivity between 231Pa and 230Th. The oceanographic influences on sediment 231Pa/230Th include (i) changes in 231Pa export by the circulation and/or in water mass 231Pa and/or 230Th activities, and (ii) changes in the chemical composition and/or vertical flux of particles. There is a need for distinguishing between the influence of particle chemical composition (e.g., biogenic opal) and that of circulation. Coverage of sediment 231Pa/230Th data is very poor. There is also a strong need for much more water column observations for better understanding the relation between 231Pa/230Th in sediment and the concentrations of 231Pa and 230Th in water (e.g., in order to better understand the effects of particle chemical composition).
The workshop participants recognized the need for standardisations and future updates of the LGM datasets. The Glacial Ocean Atlas website will provide a forum for information exchange, links to data products and key papers, as well as templates for submitting and archiving new data.
Reference
Otto-Bliesner, B.L., C. D. Hewitt, T. M. Marchitto, E. Brady, A. Abe-Ouchi, A. M. Crucifix, S. Murakami, S.L. Weber (2007), Last Glacial Maximum ocean thermohaline circulation: PMIP2 model intercomparsions and data constraints, Geophys. Res. Lett., 34, L12706, doi:10.1029/2007GL029475.
Fig. 1. Coverage of deep-ocean proxy data. Location map for the distribution of existing paleoceanographic records for the LGM. The five different symbols indicate the five different proxies that can potentially be used for reconstructing the glacial deep ocean circulation.
