Showing papers by "David W. Lea published in 2014"
TL;DR: In this article, the effect of calcite dissolution on Mg/Ca ratios of eight planktonic foraminiferal species from the South China Sea sediment surface was investigated.
Abstract: Mg/Ca ratios of planktonic foraminiferal tests are important tools for reconstructing past ocean temperatures at different levels of the upper water column. Yet numerous studies suggest a significant influence of calcite dissolution on Mg/Ca ratios lowering their initial signal recorded within a planktonic foraminiferal habitat. To determine the effect of dissolution, this study presents Mg/Ca ratios of eight planktonic foraminiferal species from the South China Sea sediment surface. Continuously decreasing with increasing water depth, the Mg/Ca ratios also decrease with calcite-saturation states close to and below saturation (bottom water Delta[CO32-] 40 mu mol kg(-1)). This preservation pattern compares well with examples of Mg/Ca dissolution from the tropical Atlantic Ocean and is independent of the foraminiferal species. Merging a global data set by separate normalization of 79 Mg/Ca data sets from the Pacific, Atlantic, and Indian Oceans, which removes thermal differences between the ocean regions and foraminiferal species, enabled us to quantify a global decrease in planktonic foraminiferal Mg/Ca ratios of 0.054 +/- 0.019 mu mol mol(-1) per mol kg(-1) below a critical threshold for dissolution of 21.3 +/- 6.6 mu mol kg(-1). The absolute decline in Mg/Ca ratios, which is similar for all species, affects temperature estimates from (sub-)thermocline species more strongly than those from shallow dwellers. The water depth of this critical threshold in the global oceans shoals from >3.5 km in the North Atlantic to <0.5 km in the North Pacific based on calculations of the global calcite-saturation state from 6321 hydrographic stations. Above this critical threshold Mg/Ca ratios are well preserved, and paleotemperature estimates are broadly unaffected by dissolution.
99 citations
01 Jan 2014
TL;DR: A remarkable amount of progress has been made in the last two decades on three new or revived paleothermometric approaches, each of which work particularly well in the tropics as mentioned in this paper.
Abstract: Geochemists have come up with a remarkable array of paleotemperature proxies in marine carbonates. These proxies work in diverse oceanic settings, in different organisms, in different parts of the water column, and on varied timescales. Several pairs of proxies, such as Mg/Ca and oxygen isotopes in foraminifera and Sr/Ca and oxygen isotopes in corals, amplify each other when applied together because they allow for the determination of seawater oxygen isotope composition, a proxy for salinity. A remarkable amount of progress has been made in the last two decades on three new or revived paleothermometric approaches, each of which work particularly well in the tropics: oxygen isotopes in corals, Mg/Ca in foraminifera, and Sr/Ca in corals. This progress has expanded the importance of geochemistry in paleoclimate research and refocused its goals toward determining the temporal and spatial history of oceanic temperatures. Although many problems and challenges remain, and although none of the available proxies work perfectly, it is clear that recent research progress has elevated geochemical paleothermometers to a fundamental role in quantitative paleoclimate research.
52 citations
TL;DR: Foraminiferal Ba/Ca and Mg/Ca time series are used to reconstruct hydrological and thermal changes over southeastern African and southwestern tropical oceans as discussed by the authors, showing that relatively dry conditions throughout the middle and late Holocene epoch are accompanied by relatively cold Mozambique Channel surface water.
38 citations
TL;DR: The equatorial Pacific was warmer during the Pliocene and late Miocene than it is today and the temperature difference between the eastern and western tropical Pacific that is a fundamental characteristic of today's ocean was present during these warmer time intervals.
Abstract: Knowledge of the behavior of the tropical oceans under different climate conditions is important for understanding not only past climate change but also present and future global warming, especially given the recent finding that the cool state of the equatorial Pacific might be the cause of the current global warming hiatus ( 1 ). On page 84 of this issue, Zhang et al. evaluate the long-term evolution of tropical Pacific sea surface temperatures (SSTs) since 12 million years ago ( 2 ). They conclude that the equatorial Pacific was warmer during the Pliocene (5.3 to 2.6 million years ago) and late Miocene (12.0 to 5.3 million years ago) than it is today and that the temperature difference between the eastern and western tropical Pacific that is a fundamental characteristic of today's ocean was present (although somewhat smaller than it is today) during these warmer time intervals.
7 citations
TL;DR: The MArGO SST reconstruction is an important validation target for the modeling community because the climate forcing and response during the last Glacial Maximum (LGM) are relatively well known and allow for an assessment of climate sensitivity determinations.
Abstract: Establishing a sea surface temperature (SST) distribution for the Last Glacial Maximum (LGM) has long been a primary goal in paleoclimate research, with the CLIMAp (Climate Long range Investigation, Mapping and prediction) reconstruction standing out as a landmark accomplishment. The development of new SST proxies such as UK37' and Mg/Ca paleothermometry in the 1980s and 90s spurred the international community to re-evaluate LGM SSTs. The publication of the SST reconstruction of MArGO (Multiproxy Approach for the reconstruction of the Glacial Ocean surface) was the culmination of an international collaboration to update and improve LGM SST reconstructions (MArGO project Members 2009). The MArGO reconstruction is an important validation target for the modeling community because the climate forcing and response during the LGM are relatively well known and allow for an assessment of climate sensitivity determinations. The purpose of the COMpArE 2013 workshop was to follow up on an initial meeting of COMpArE 2012 (Comparing Ocean Models with paleo-Archives) in Bremen, Germany (18-21 March 2012) which had discussed ways to further improve the MArGO data set and how to apply it to data-model intercomparisons (Kucera et al. 2012). The MArGO SST reconstruction synthesizes both more traditional approaches (i.e. faunal transfer functions) and newer geo-chemical approaches (i.e. Mg/Ca and UK37'). But because the original MArGO data set for the tropics is still dominated by estimates based on transfer functions, the reconstructed SST changes do not optimally represent the different proxy results. The COMpArE 2013 workshop brought together observationalists and model-ers to exchange their latest insights on tropical SST proxies and the efficacy of the MArGO low latitude reconstructions. There are many reasons for re-evaluating LGM tropical SST, including the MArGO compilation itself, which displays a much stronger level of spatial heterogeneity in the tropics than has been simulated by coupled model runs (Kag-eyama et al. 2013); the validity of the SST proxies and their overall compatibility; advances in linking climate sensitivity to LGM tropical cooling in models (Hargreaves et al. 2012); downward revisions of LGM cooling based on MArGO; and advances in analyzing and attributing deglacial SST records, which are increasingly based on geochemical proxy records (Shakun et al. 2012). We identified two overarching research questions to frame the problem: • Can we reconcile UK37' and Mg/Ca data within acceptable brackets by using harmonized calibration schemes and by considering differences in habitat depth and season? If so, what was the magnitude of mean …
1 citations