Eva María Calvo

Apparent long-term cooling of the sea surface in the northeast Atlantic and Mediterranean during the Holocene.

Reconstructions of upper ocean temperature (T) during the Holocene (10–0 ka B.P.) were established using the alkenone method from seven, high accumulation sediment cores raised from the northeast Atlantic and the Mediterranean Sea (361N–751N). All these paleo-T records document an apparent long-term cooling during the last 10 kyr. In records with indication of a constant trend, the apparent cooling ranges from � 0.27 to � 0.151C kyr � 1 . Records with indication of time-variable trend show peak-to-peak amplitudes in apparent temperatures of 1.2–2.91C. A principal component analysis shows that there is one factor which accounts for a very large fraction (67%) of the total variance in the biomarker paleo-T records and which dominates these records over other potential secondary influences. Two possible contributions are (1) a widespread surface cooling, which may be associated with the transition fromthe Hypsithermal interval ( B9–5.7 ka B.P.) to the Neoglaciation (B5.7–0 ka B.P.); and (2) a change in the seasonal timing and/or duration of the growth period of alkenone producers (prymnesiophyte algae). The first contribution is consistent with many climate proxy records from the northeast Atlantic area and with climate model simulations including Milankovitch forcing. The second contribution is consistent with the divergence between biomarker and summer faunal paleo-T fromearly to late Holocene observed in two cores. Further work is necessary, and in particular the apparent discordance between biomarker and faunal T records for the relative stable Holocene period must be understood, to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records. r 2002 Elsevier Science Ltd. All rights reserved.

Identification and removal of Mn‐Mg‐rich contaminant phases on foraminiferal tests: Implications for Mg/Ca past temperature reconstructions

[1] The geochemical composition of foraminifera shells from an Ocean Drilling Program site in the Panama Basin has been analyzed by several analytical techniques (LA-ICP-MS, ICP-MS, XRD, SEM, EDX) in order to identify and evaluate the occurrence of contaminant phases which may bias paleoenvironmental reconstructions. LA-ICP-MS results on uncleaned tests indicate the presence of MnMg-rich contaminant phases at the inner surfaces of the foraminiferal shells (which have Mn/Ca ratios up to 400 mmol mol 1 and Mg/Ca ratios up to 50 mmol mol 1 ). We have rigorously assessed the ability of different cleaning protocols to remove these contaminant phases and have obtained satisfactory results only when a reductive step is included. The analysis of cleaning residuals collected after each of the different cleaning steps applied reveals that high Mn values are associated with at least two different contaminant phases, of which only one is linked to high Mg values. XRD analysis further reveals that the Mn-Mg-rich phase is the Ca-Mn-Mg carbonate kutnahorite (Ca(Mn, Mg)(CO3)2). Our results demonstrate that the presence of kutnahorite-like minerals can bias Mg/Ca ratios toward higher values (by 7–36%) and lead to significant overestimation of past seawater temperatures (by 0.9 up to 6.2� C, in the case of these Panama Basin samples). Components: 11,867 words, 13 figures, 3 tables.

Paleo-perspectives on ocean acidification

The anthropogenic rise in atmospheric CO(2) is driving fundamental and unprecedented changes in the chemistry of the oceans. This has led to changes in the physiology of a wide variety of marine organisms and, consequently, the ecology of the ocean. This review explores recent advances in our understanding of ocean acidification with a particular emphasis on past changes to ocean chemistry and what they can tell us about present and future changes. We argue that ocean conditions are already more extreme than those experienced by marine organisms and ecosystems for millions of years, emphasising the urgent need to adopt policies that drastically reduce CO(2) emissions.

Antarctic deglacial pattern in a 30 kyr record of sea surface temperature offshore South Australia

[1] Comparison of ice cores from Greenland and Antarctica shows an asynchronous two-step warming at these high latitudes during the Last Termination. However, the question whether this asynchrony extends to lower latitudes is unclear mainly due to the scarcity of paleorecords from the Southern Hemisphere. New data from a marine core collected off South Australia (� 36S) allows a detailed reconstruction of sea-surface temperatures over the Last Termination. This confirms the existence of an Antarctic-type deglacial pattern and shows no indication of cooling associated with the Northern Hemisphere YD event. The SST record also provides a new comparison with the more extensive paleoclimatic data available from continental Australia. This shows a strong climatic link between onshore and offshore records for Australia and to Southern Hemisphere paleorecords. We also show a progressive SST drop over the last � 6.5 kyr not seen before for the Australian region. Citation: Calvo, E., C. Pelejero, P. De Deckker, and G. A. Logan (2007), Antarctic deglacial pattern in a 30 kyr record of sea surface temperature offshore South Australia, Geophys. Res. Lett., 34, L13707, doi:10.1029/2007GL029937.

New insights into the glacial latitudinal temperature gradients in the North Atlantic. Results from UK′37 sea surface temperatures and terrigenous inputs

Abstract Sea surface temperatures (SST) and input of continental materials have been reconstructed from the study of the long-chain alkenones and n-alkanes, respectively, in a core located in the western side of the Mid-Atlantic Ridge (MD952037, 37°05′N 32°02′W, 2630 m depth). Both the long- and short-term variability recorded by the temperatures and the planktonic δ18O are basically the same over the last 280 kyr, showing a clear glacial/interglacial evolution. Comparison with core SU90/08, located only 6° north and directly influenced by the cold polar waters associated with the polar front during glacial times, revealed different climatic conditions during glacial periods at both locations. Whereas core MD952037 recorded similar SST values during the last two glacial periods (ca. 14–15°C), the northern core displayed colder conditions during isotopic stage 2 (8–10°C) than in stage 6 (13–15°C). These results indicate the existence of a well-developed steep north–south gradient between 37 and 43°N during the last glacial period but not during stage 6, which suggests a southern expansion of the polar front during the last glacial maximum.

Characterization of contaminant phases in foraminifera carbonates by electron microprobe mapping

The advent of new microanalytical techniques such as electron microprobe mapping (EMP) and laser ablation microsamplers coupled to mass spectrometers (LA-ICP-MS) provides a new array of possibilities to explore in great detail the trace elements distribution in foraminiferal carbonates. Here we apply these techniques to characterize diagenetic phases present in foraminiferal shells from Ocean Drilling Program Site 1240 in the Panama Basin, a region characterized by the presence of manganese-rich minerals in the sediments. The combined application of these techniques allows us to characterize the elemental and spatial distribution on the surface and across the foraminiferal shells. Results illustrate the presence of at least two different Mn-rich contaminant phases in the foraminiferal carbonates: Mn-rich carbonates and ferromanganese oxides. Elemental maps also highlight the relevance of the foraminifera shell texture and porosity in the distribution and formation of these contaminant phases. In the case of Neogloboquadrina dutertrei, Mn phases form a continuous thin layer in the inner part of the chambers, whereas for Globigerinoides ruber, Mn phases have a rather patchy distribution and are usually found within the pores. Significant high magnesium concentrations are always associated with these Mn-rich phases. These new findings support the need of removing these Mn-rich contaminant phases in order to measure accurately the Mg/Ca ratios in the foraminifera shell and therefore obtain reliable Mg/Ca paleotemperature reconstructions.

Turnover time of fluorescent dissolved organic matter in the dark global ocean

Marine dissolved organic matter (DOM) is one of the largest reservoirs of reduced carbon on Earth. In the dark ocean (>200 m), most of this carbon is refractory DOM. This refractory DOM, largely produced during microbial mineralization of organic matter, includes humic-like substances generated in situ and detectable by fluorescence spectroscopy. Here we show two ubiquitous humic-like fluorophores with turnover times of 435±41 and 610±55 years, which persist significantly longer than the ~350 years that the dark global ocean takes to renew. In parallel, decay of a tyrosine-like fluorophore with a turnover time of 379±103 years is also detected. We propose the use of DOM fluorescence to study the cycling of resistant DOM that is preserved at centennial timescales and could represent a mechanism of carbon sequestration (humic-like fraction) and the decaying DOM injected into the dark global ocean, where it decreases at centennial timescales (tyrosine-like fraction).

Sea surface paleotemperature errors in UK′ 37 estimation due to alkenone measurements near the limit of detection

Major errors (> 0.5°C) are produced in paleotemperature estimation with the C37 alkenone method when the amount of the diunsaturated or triunsaturated species, C37:2 and C37:3, respectively, approaches the limit of detection. These errors are more commonly encountered with C37:3 because of its higher adsorption to gas chromatographic (GC) columns. They are particularly relevant at low sedimentary C37 alkenone concentrations and may easily increase to temperature differences of 2°–4°C depending on the UK′37 values and the ratio between the C37:3 signal and limit of detection. Mathematical expressions for the description of the analytical constraints derived from this detection limit problem are given. A quality assurance guideline based on sample dilution (three times) and calculation of the sea surface temperature error is proposed for assessment of reliable measurements. Reevaluation of literature data in the light of the equations reported in the present study suggest that some previously reported alkenone geochemical effects, such as variation in UK′37 temperatures due to postdepositional oxidation, may reflect deviations in the GC measurements due to changes in C37 alkenone concentration.

Eastern Equatorial Pacific productivity and related-CO2 changes since the last glacial period

Understanding oceanic processes, both physical and biological, that control atmospheric CO2 is vital for predicting their influence during the past and into the future. The Eastern Equatorial Pacific (EEP) is thought to have exerted a strong control over glacial/interglacial CO2 variations through its link to circulation and nutrient-related changes in the Southern Ocean, the primary region of the world oceans where CO2-enriched deep water is upwelled to the surface ocean and comes into contact with the atmosphere. Here we present a multiproxy record of surface ocean productivity, dust inputs, and thermocline conditions for the EEP over the last 40,000 y. This allows us to detect changes in phytoplankton productivity and composition associated with increases in equatorial upwelling intensity and influence of Si-rich waters of sub-Antarctic origin. Our evidence indicates that diatoms outcompeted coccolithophores at times when the influence of Si-rich Southern Ocean intermediate waters was greatest. This shift from calcareous to noncalcareous phytoplankton would cause a lowering in atmospheric CO2 through a reduced carbonate pump, as hypothesized by the Silicic Acid Leakage Hypothesis. However, this change does not seem to have been crucial in controlling atmospheric CO2, as it took place during the deglaciation, when atmospheric CO2 concentrations had already started to rise. Instead, the concomitant intensification of Antarctic upwelling brought large quantities of deep CO2-rich waters to the ocean surface. This process very likely dominated any biologically mediated CO2 sequestration and probably accounts for most of the deglacial rise in atmospheric CO2.

Insolation dependence of the southeastern subtropical Pacific sea surface temperature over the last 400 kyrs

The present study describes the first sea surface temperature (SST) reconstruction in the southeastern Subtropical Pacific Ocean, offshore the South American coast. The obtained record encompasses the last 400 kyr and follows the characteristic glacial/interglacial pattern defined by global ice volume. However, SST leads the δ18O isotopic record reflecting the role of the low latitudes areas in driving climate change. SST in the Holocene is lower by about 0.5–0.8°C than maximal SST in stages 5e, 7, 9 and 11. SST in stages 2–4 is lower by about 0.6–1.3°C than minimal SST in stages 6, 8 and 10. These features are similar to SST records obtained in the South Atlantic Ocean pointing to a general inter-basinal behaviour at these low latitudes. For most of the record, the observed long-term SST evolution is well correlated with the orbital parameter of eccentricity, which modulates the insolation at low latitudes. However, at low eccentricity values (Stages 11–9 and Termination I), SST is driven by obliquity, exhibiting a dependence from high latitude climatic responses.