Belen Martrat

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.

Interglacials of the last 800,000 years

Interglacials, including the present (Holocene) period, are warm, low land ice extent (high sea level), end-members of glacial cycles. Based on a sea level definition, we identify eleven interglacials in the last 800,000 years, a result that is robust to alternative definitions. Data compilations suggest that despite spatial heterogeneity, Marine Isotope Stages (MIS) 5e (last interglacial) and 11c (~400 ka ago) were globally strong (warm), while MIS 13a (~500 ka ago) was cool at many locations. A step change in strength of interglacials at 450 ka is apparent only in atmospheric CO2 and in Antarctic and deep ocean temperature. The onset of an interglacial (glacial termination) seems to require a reducing precession parameter (increasing Northern Hemisphere summer insolation), but this condition alone is insufficient. Terminations involve rapid, nonlinear, reactions of ice volume, CO2, and temperature to external astronomical forcing. The precise timing of events may be modulated by millennial-scale climate change that can lead to a contrasting timing of maximum interglacial intensity in each hemisphere. A variety of temporal trends is observed, such that maxima in the main records are observed either early or late in different interglacials. The end of an interglacial (glacial inception) is a slower process involving a global sequence of changes. Interglacials have been typically 10–30 ka long. The combination of minimal reduction in northern summer insolation over the next few orbital cycles, owing to low eccentricity, and high atmospheric greenhouse gas concentrations implies that the next glacial inception is many tens of millennia in the future.

A global multiproxy database for temperature reconstructions of the Common Era

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850–2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.

Land-ocean changes on orbital and millennial time scales and the penultimate glaciation

Past glacials can be thought of as natural experiments in which variations in boundary conditions influenced the character of climate change. However, beyond the last glacial, an integrated view of orbital- and millennial-scale changes and their relation to the record of glaciation has been lacking. Here, we present a detailed record of variations in the land-ocean system from the Portuguese margin during the penultimate glacial and place it within the framework of ice-volume changes, with particular reference to European ice-sheet dynamics. The interaction of orbital- and millennial-scale variability divides the glacial into an early part with warmer and wetter overall conditions and prominent climate oscillations, a transitional mid-part, and a late part with more subdued changes as the system entered a maximum glacial state. The most extreme event occurred in the mid-part and was associated with melting of the extensive European ice sheet and maximum discharge from the Fleuve Manche river. This led to disruption of the meridional overturning circulation, but not a major activation of the bipolar seesaw. In addition to stadial duration, magnitude of freshwater forcing, and background climate, the evidence also points to the influence of the location of freshwater discharges on the extent of interhemispheric heat transport.

Climatic dependence of the organic matter contributions in the north eastern Norwegian Sea over the last 15,000 years

Abstract Lipids are used for the evaluation of the different organic matter contributions in the north eastern Norwegian Sea (M23258 site, 75°N, 14°E) over the last 15,000 years. Development of a mass balance model based on the down-core quantification of the C 37 alkenones, the odd carbon numbered n -alkanes (A odd ) and the unresolved complex mixture of hydrocarbons (UCM) has allowed three main organic matter inputs involving marine, continental and ancient reworked organic matter to be recognized. The model shows good agreement between measured and reconstructed TOC values. Similarly, a strong parallelism is observed between predicted components such as marine TOC and carbonate content, which was determined independently. Representation of the model results within a time scale based on 15 AMS- 14 C measurements shows that the main changes in organic matter constituents are coincident with the major climatic events of the last 15,000 years. Thus, the predominance of reworked organic matter is characteristic of Termination Ia (up to 70%), continental organic matter was dominant during the Bolling-Allerod (B-A) and Younger Dryas (YD) periods (about 85%) and a strong increase of marine organic matter occurred in the Holocene (between 50 and 75%). This agreement reflects the main hydrographic changes that determined the deposition of sedimentary materials between 0 and 15 cal ka: ice-rafted detritus from the Barents continental platform, ice-melting waters from the Arctic fluvial system discharging into the Barents Sea and dominance of North Atlantic currents, respectively. In this respect, the high-resolution down-core record resulting from the mass balance and lipid measurements allows for the identification of millennial scale events such as the increase of reworked organic matter at the final retreat of the Barents ice sheet at the end of the deglaciation period (Termination Ib).

Anatomy of Heinrich Layer 1 and its role in the last deglaciation

This research used data acquired at the XRF Core Scanner Lab at the MARUM–Center for Marine Environmental Sciences, University of Bremen, Germany. This research used samples provided by the International Ocean Discovery Program (IODP). Funding for this research was provided by the UK Natural Environmental Research Council (NERC) to Hodell. The NERC Radiocarbon Facility supported two radiocarbon dates, and Wally Broecker generously supported the remainder with funding from the Comer Family Foundation. Research by Rodriguez-Tovar and Dorador was financed by Project CGL2015-66835-P. B.M. acknowledges support from the CSIC-Ramon y Cajal postdoctoral programme RYC-2013-14073. J.F.E. would like to acknowledge funding under ERC Advanced grant 320750- Nanopaleomagnetism.

Atmospheric patterns driving Holocene productivity in the Alboran Sea (Western Mediterranean): A multiproxy approach

High-resolution paleoproductivity variations have been reconstructed in a productive cell in the Alboran Sea for the Holocene. Fossil coccolithophore assemblages have been studied along with the U 37 K ′ -estimated sea-surface temperature (SST) and other paleoenvironmental proxies. The appearance of this cell is suggested at 7.7 ka cal. BP and was linked to the establishment of the western anti-cyclonic gyre. From that time until the present, the nannofossil accumulation rate of Florisphaera profunda has revealed successive episodes of weakening and strengthening of upwelling conditions in the Alboran Sea that have been simultaneous to changes in Western Mediterranean Deep Water (WMDW) formation in the Gulf of Lions. A two-phase scenario operating at millennial–centennial time scale is proposed to explain this climatic and oceanographic variability: (1) coeval with more arid climate conditions, weaker northerlies or north-westerlies blowing over the Gulf of Lions would have triggered a slackening of WMDW formation. This together with a minor Atlantic Jet (AJ) inflowing into the Alboran Sea would have led to less vertical mixing and, hence, a more stable water column in the study area; (2) wetter climate conditions would have prevailed in the region, while stronger northerlies or north-westerlies would have enabled WMDW reinforcement in the Gulf of Lions simultaneous to an intensification of the AJ that migrated southward. This would have increased vertical mixing, intensifying upwelling conditions in the study area. Here, the winter North Atlantic Oscillation (NAO) is considered to be an important forcing mechanism for this variability, influencing WMDW formation, which in turn has been linked to short-term productivity variations during the last 7.7 ka in the Alboran Sea.

The response of SST to insolation and ice sheet variability from MIS 3 to MIS 11 in the northwestern Mediterranean Sea (Gulf of Lions)

Here we present a sea surface temperature (SST) record based on the Uk′37 index from the PRGL1 borehole (Promess1) drilled on the upper slope of the Gulf of Lions (GL). This is the first continuous and high-resolution record in the northwestern Mediterranean Sea from marine oxygen isotope stage 3 (MIS) 3 to MIS 11. Due the location of the GL, the SST proxy can be considered to be a reliable tool to study the climate link between high latitude and midlatitude. During glacial inceptions, the northern ice sheet signal via cold northwesterly winds was first recorded in our study area in comparison with southern locations, highlighting the strong sensitivity of this location to high-latitude dynamics. Moreover, the amplitude of the millennial-scale variability in the western Mediterranean basin seems to be the result of both ice sheet and insolation variability.

Mediterranean circulation perturbations over the last five centuries: Relevance to past Eastern Mediterranean Transient-type events

The Eastern Mediterranean Transient (EMT) occurred in the Aegean Sea from 1988 to 1995 and is the most significant intermediate-to-deep Mediterranean overturning perturbation reported by instrumental records. The EMT was likely caused by accumulation of high salinity waters in the Levantine and enhanced heat loss in the Aegean Sea, coupled with surface water freshening in the Sicily Channel. It is still unknown whether similar transients occurred in the past and, if so, what their forcing processes were. In this study, sediments from the Sicily Channel document surface water freshening (SCFR) at 1910 ± 12, 1812 ± 18, 1725 ± 25 and 1580 ± 30 CE. A regional ocean hindcast links SCFR to enhanced deep-water production and in turn to strengthened Mediterranean thermohaline circulation. Independent evidence collected in the Aegean Sea supports this reconstruction, showing that enhanced bottom water ventilation in the Eastern Mediterranean was associated with each SCFR event. Comparison between the records and multi-decadal atmospheric circulation patterns and climatic external forcings indicates that Mediterranean circulation destabilisation occurs during positive North Atlantic Oscillation (NAO) and negative Atlantic Multidecadal Oscillation (AMO) phases, reduced solar activity and strong tropical volcanic eruptions. They may have recurrently produced favourable deep-water formation conditions, both increasing salinity and reducing temperature on multi-decadal time scales.

Removal of bias in C 37 alkenone-based sea surface temperature measurements by high-performance liquid chromatography fractionation

Sea surface temperature determinations based on marine sedimentary C37 alkenone distributions have provided a wealth of data for paleoclimatic studies, including those performed at high resolution. The success of this approach results from several characteristics of alkenone compounds, e.g. their geochemical properties (such as unequivocal synthesis by certain widespread haptophyte algae, plus chemical stability/preservation of the original alkenone distributions during sedimentation), and their analytical properties (such as fast clean up procedures using alkaline hydrolysis of sediment extracts, followed by robust instrumental methods allowing large scale sample processing). Here we show that, in sediments under the influence of continental inputs, coelution of these compounds with cholest-5-enyl 3β-undecenyl ether and 24-methylcholesta-5,22-dienyl 3β-undecenyl ether deviate the SST measurements despite alkaline hydrolysis. Here, we report a new high performance liquid chromatrography fractionation method which eliminates these interfering compounds and gathers all the alkenones into a single fraction. These fractions can then be analysed by gas chromatography as in the initial approach, providing large amounts of data as required in high resolution studies.