Changes In Thermohaline Circulation Research Articles

Deep learning-based gap filling for near real-time seamless daily global sea surface salinity using satellite observations

Sea surface salinity (SSS) provides crucial information about ocean environments, influencing global hydrological cycles, thermohaline circulation, and climate change. Although L-band passive microwave radiometers have provided satellite-based SSS data, there are gaps due to the limited daily coverage of the sensors. This study proposes a U-Net-based spatial gap-filling model for global SSS using Soil Moisture Active Passive (SMAP) satellite data. The proposed model utilizes SSS swath data from the target and past days to generate daily global SSS maps with full coverage, incorporating only past temporal information. Additionally, bias-corrected data using gradient-boosted regression trees (GBRT) are employed to reduce inherent errors in the SMAP SSS data. We designed 24 schemes using data from the past 3, 5, and 7 days for both GBRT-corrected and original SMAP SSS data, with one to four times oversampling for low-salinity water, where the number of samples is significantly small. Validation results using masked-out pixels indicate that the gap-filling models with GBRT-corrected SSS data from the past 3 and 5 days and four times oversampling yielded the best performance, with root mean square errors (RMSEs) of 0.388 and 0.413 psu, respectively. Compared with the in situ Argo data for 2020, the RMSEs were 0.237 and 0.241 psu for the two models, respectively, significantly outperforming the SMAP Level 3 8-day SSS, which requires future data (RMSE of 0.456 psu). Notably, these models successfully filled gaps over coastal areas where low SSS (<30 psu) fluctuated due to freshwater discharge from inland areas. This study proposes an effective, novel gap-filling method for generating bias-corrected global daily SSS without delay, meeting operational needs. Moreover, the proposed gap-filling framework can be applied to other environmental swaths or even grid datasets.

Review of the indexes to assess the ecological quality of coralligenous reefs: towards a unified approach

There is an urgent need to better understand the stressors, namely heatwaves, changes in thermohaline circulation and mucilage events, that are rapidly re-shaping bioconstructions, such as coralligenous assemblages. This calls for increased monitoring efforts in these invaluable habitats that will improve our understanding of the resistance and resilience of bioconstructions. Since 2009, 16 indexes have been designed to assess the ecological quality of Mediterranean coralligenous reefs. The main objective of this work is to propose a framework to support the development of a shared, cost-effective, and practical index to assess the status of the coralligenous biocenosis. To achieve this, studies conceiving these 16 indexes were reviewed: comparing their objectives, metrics, and applied methodologies. A standardized nomenclature of anthropogenic pressures is supplied, using, when possible, definitions from the European Habitat Directive, Marine Strategy Framework Directive and Water Framework Directive. Additionally, given the unprecedented climatic conditions, we highlight that a common index should give particular attention to the response of the coralligenous to thermal stress and mucilage. A list of priority anthropogenic pressures/environmental stressors and relative indicators and metrics are suggested. This review stresses the urgency to align the methodologies at basin scale and highlights the pros and cons of the preexisting indexes that must be considered in the design of a new, shared procedure to evaluate the status of coralligenous assemblages.

Changes in North Atlantic deep-water oxygenation across the Middle Pleistocene Transition.

The oxygen concentrations of oceanic deep-water and atmospheric carbon dioxide (pCO2) are intrinsically linked through organic carbon remineralization and storage as dissolved inorganic carbon in the deep sea. We present a high-resolution reconstruction of relative changes in oxygen concentration in the deep North Atlantic for the past 1.5 million years using the carbon isotope gradient between epifaunal and infaunal benthic foraminifera species as a proxy for paleo-oxygen. We report a significant (>40 micromole per kilogram) reduction in glacial Atlantic deep-water oxygenation at ~960 thousand to 900 thousand years ago that coincided with increased continental ice volume and a major change in ocean thermohaline circulation. Paleo-oxygen results support a scenario of decreasing deep-water oxygen concentrations, increased respired carbon storage, and a reduction in glacial pCO2 across the Middle Pleistocene Transition.

Feedback processes modulating the sensitivity of Atlantic thermohaline circulation to freshwater forcing timescales

AbstractPaleo proxy records indicate that abrupt changes in thermohaline circulation (THC) were induced by rapid meltwater discharge from retreating ice sheets. Such abrupt changes in the THC have been understood as a hysteresis behavior of nonlinear system. Previous studies, however, primarily focused on a near-static hysteresis under fixed or slowly varying freshwater forcing (FWF), reflecting the equilibrated response of the THC. This study aims to improve the current understanding of transient THC responses under rapidly varying forcing and its dependency on forcing timescales. The results simulated by an Earth system model suggest that the bifurcation is delayed as the forcing timescale is shorter, causing the Atlantic meridional overturning circulation collapse (recovery) to occur at higher (lower) FWF values. The delayed shutdown/recovery occurs because bifurcation is determined not by the FWF value at the time but by the total amount of freshwater remaining over the THC convection region. The remaining freshwater amount is primarily determined by the forcing accumulation (i.e., time-integrated FWF), which is modulated by the freshwater/salt advection by ocean circulations and freshwater flux by the atmospheric hydrological cycle. In general, the latter is overwhelmed by the former. When the forced freshwater amount is the same, the modulation effect is stronger under slowly varying forcing because more time is provided for the feedback processes.

Changes in western Mediterranean thermohaline circulation in association with a deglacial Organic Rich Layer formation in the Alboran Sea

The accumulation of an Organic Rich Layer (ORL) during the last deglaciation in the Alboran Sea (western Mediterranean Sea) and its link to changes in deep and intermediate water circulation are here investigated. Benthic foraminiferal assemblages and the shallow infaunal foraminifer Uvigerina peregrina δ13C record support the establishment of sustained high organic matter fluxes, and thus eutrophic conditions at the sea floor, during the late phase of the ORL (Younger Dryas to early Holocene periods). Since organic matter fluxes were lower (mesotrophic conditions) during the Bølling-Allerød period, they cannot be solely responsible for the ORL initiation. Geochemical, sedimentological and micropalaeontological proxies support a major weakening of the deep-water convection in the Gulf of Lion as the main driver for the development of poorly-ventilated conditions from intermediate depths (946 m) to the deep western Mediterranean basin that promoted the beginning of the ORL deposition. Nevertheless, a better ventilation at intermediate depths was established during the late ORL, while the deep basin remained poorly ventilated. We propose that our data reflect the arrival of a new better-ventilated intermediate water mass analogue to the current Levantine Intermediate Water (LIW) and/or a new intermediate water mass from the Gulf of Lion. The ultimate source of this water mass needs to be further explored but chronologies of the changes recorded here indicate that intermediate and deep ventilation phases were decoupled between the western and eastern Mediterranean basins during the deglaciation and early-middle Holocene.

Low frequency variability and sensitivity of the Atlantic meridional overturning circulation in selected IPCC climate models

Abstract The structure of main modes of the decadal and multidecadal variability of the Atlantic meridional overturning circulation (AMOC) is analyzed for the climate models INM-CM5 (INM RAS), CCSM4 (NCAR), MPI-LR and MPI-MR (MPI). It is shown that oscillations with characteristic periods of 25–35 and 50–70 years are recognized in the models, and the corresponding spatial structures are variations of the intensity and position of the AMOC. Relations connecting changes of thermohaline circulation with external impacts on the system are constructed for the above models. The analysis of stability of calculations relative to the length of data series used for calculations is performed. The optimal impacts leading to the greatest response of the AMOC and influence functions causing its response along the leading modes of low-frequency variability are constructed. The optimal impacts have the form of density anomalies localized in the North Atlantic, and the structures of the corresponding responses are dominated by leading low-frequency modes of variability. The structure of optimal impacts varies greatly from model to model.

Biology and distribution of hyperiids in the Sea of Okhotsk

Long-term studies in the Sea of Okhotsk (1986–2012) demonstrated that hyperiids account for a small portion of zooplankton, making up only from 1.0 to 5.3% by weight. The minimum hyperiid biomass was observed in the coastal zone. The hyperiid biomass in the open water shelf community increased from spring to autumn. In the Sea of Okhotsk, hyperiids are represented by ten species, among which the most abundant in the northern regions (mainly in Shelikhov Gulf) is Themisto libellula, while the most abundant in other regions is T. pacifica. The distribution of T. pacifica in different seasons showed that the maximum biomass was concentrated in the deep-water zone. The range of T. libellula in the Sea of Okhotsk is mostly limited to the Shelikhov Gulf, but in certain years its habitation area can expand. In warm years with low ice coverage, the maximum expansion of T. libellula to the west in the shelf zone of the northern part of the Sea of Okhotsk was observed. Since variability of salinity and temperature in the northern part of the Sea of Okhotsk may be fatal for the cryophilic stenohaline T. libellula species, forecasted changes in thermohaline circulation will make it possible to predict the population dynamics of this important species.

Circulation and oxygen cycling in the Mediterranean Sea: Sensitivity to future climate change

Climate change is expected to increase temperatures and decrease precipitation in the Mediterranean Sea (MS) basin, causing substantial changes in the thermohaline circulation (THC) of both the Western Mediterranean Sea (WMS) and Eastern Mediterranean Sea (EMS). The exact nature of future circulation changes remains highly uncertain, however, with forecasts varying from a weakening to a strengthening of the THC. Here, we assess the sensitivity of dissolved oxygen (O2) distributions in the WMS and EMS to THC changes using a mass balance model, which represents the exchanges of O2 between surface, intermediate and deep water reservoirs, and through the Straits of Sicily and Gibraltar. Perturbations spanning the ranges in O2 solubility, aerobic respiration kinetics and THC changes projected for the year 2100 are imposed to the O2 model. In all scenarios tested, the entire MS remains fully oxygenated after 100 years; depending on the THC regime, average deep-water O2 concentrations fall in the ranges 151-205 and 160-219 µM in the WMS and EMS, respectively. On longer timescales (>1000 years), the scenario with the largest (> 74%) decline in deep-water formation rate leads to deep-water hypoxia in the EMS but, even then, the WMS deep-water remains oxygenated. In addition, a weakening of THC may result in a negative feedback on O2 consumption as supply of labile dissolved organic carbon to deep-water decreases. Thus, it appears unlikely that climate-driven changes in THC will cause severe O2 depletion of the deep-water masses of the MS in the foreseeable future. This article is protected by copyright. All rights reserved.

Millennial-Scale Interaction between Ice Sheets and Ocean Circulation during Marine Isotope Stage 100

Waxing/waning of the ice sheets and the associated change in thermohaline circulation have played an important role in global climate change since major continental ice sheets appeared in the northern hemisphere about 2.75 million years ago. In the earliest glacial stages, however, establishment of the linkage between ice sheet development and ocean circulation remain largely unclear. Here we show new high-resolution records of marine isotope stage 100 recovered from deep-sea sediments on the Gardar Drift, in the subpolar North Atlantic. Results of a wide range of analyses clearly reveal the influence of millennial-scale variability in iceberg discharge on ocean surface condition and bottom current variability in the subpolar North Atlantic during marine isotope stage 100. We identified eight events of ice-rafted debris, which occurred mostly with decreases in sea surface temperature and in current components indicating North Atlantic Deep Water. These decreases are interpreted by weakened deep water formation linked to iceberg discharge, similarly to observations from the last glacial period. Dolomite fraction of the ice-rafted events in early MIS 100 like the last glacial Heinrich events suggests massive collapse of the Laurentide ice sheet in North America. At the same time, our early glacial data suggest differences from the last glacial period: absence of 1470-year periodicity in the interactions between ice sheets and ocean, and northerly shift of the ice-rafted debris belt. Our high-resolution data largely improve the picture of ice-sheet/ocean interactions on millennial time scales in the early glacial period after major Northern Hemisphere glaciation.

Model–proxy comparison for overshoot phenomenon of Atlantic thermohaline circulation at Bølling–Allerød

The overshoot phenomenon of the Atlantic thermohaline circulation (THC) is a transient climate response to meltwater forcing and could induce intense climate change by increasing the magnitudes of Atlantic THC changes at the end of meltwater discharges. This phenomenon was formally presented with the successfully simulated Bolling–Allerod (BA) event in the first transient simulation of the last deglaciation with fully coupled model NCAR-CCSM3 (TraCE-21K). Currently, not all proxy records of Atlantic THC support the occurrence of the THC overshoot at BA. Commonly used THC proxy from Bermuda Rise (GGC5) does not exhibit THC overshoot at BA but other proxies such as TTR-451 at Eirik Drift do. How to interpret this regional discrepancy of proxy records is a key question for the validation of the Atlantic THC overshoot at BA. Here, we show that the vigor of deep circulation varies regionally during the Atlantic THC overshoot at BA in TraCE-21K simulation, and this regional discrepancy in the simulation is consistent with that in the marine sediment records in North Atlantic. The consistent model–proxy evidence supports the occurrence of Atlantic THC overshoot at BA.

Dense intermediate water outflow from the Cretan Sea: A salinity driven, recurrent phenomenon, connected to thermohaline circulation changes

AbstractData collected from different platforms in the Cretan Sea during the 2000s decade present evidence of gradually increasing salinity in the intermediate and deep intermediate layers after the middle of the decade. The observed gradual salt transport toward the deeper layers indicates contributions of dense water masses formed in various Aegean Sea subbasins. The accumulation of these saline and dense water masses in the Cretan Sea finally led to outflow from both Cretan Straits, with density greater than typical Levantine/Cretan Intermediate water but not dense enough to penetrate into the deep layers of the Eastern Mediterranean. We name this outflowing water mass as dense Cretan Intermediate Water (dCIW). A retrospective analysis of in situ data and literature references during the last four decades shows that similar events have occurred in the past in two occasions: (a) in the 1970s and (b) during the Eastern Mediterranean Transient (EMT) onset (1987–1991). We argue that these salinity‐driven Aegean outflows are mostly attributed to recurrent changes of the Eastern Mediterranean upper thermohaline circulation that create favorable dense water formation conditions in the Aegean Sea through salinity preconditioning. We identify these phenomena as “EMT‐like” events and argue that in these cases internal thermohaline mechanisms dominate over atmospheric forcing in dense water production. However, intense atmospheric forcing over an already salinity preconditioned basin is indispensable for creating massive deep water outflow from the Cretan Sea, such as the EMT event.

Land–sea climatic variability in the eastern North Atlantic subtropical region over the last 14,200 years: Atmospheric and oceanic processes at different timescales

High-temporal resolution analysis of different climatic tracers (pollen, foraminiferal-based winter sea surface temperature (SST), benthic foraminiferal δ18O) from marine core MD95-2042, retrieved off SW Iberia, allows us to directly compare, without any chronological ambiguity, Mediterranean vegetation and eastern North Atlantic winter SST changes for the last 14.2 kyr. We identify on land and in the ocean several climatic phases such as the end of the warm and humid Bølling–Allerød, the cold and dry Younger Dryas, and the warm and humid Holocene with the Mediterranean forest (MF) optimum between 9.6 and 8.1 kyr. This record shows that, at multi-centennial timescale (~800 years), declines in forest cover generally related to dry and cool periods in southern Iberia are synchronous with cold SST in the eastern part of the North Atlantic subtropical gyre. At multi-centennial timescale, changes in thermohaline circulation, via freshwater content fluctuations, appear to be responsible for the coupling between dryness in Iberia and SST cooling in eastern North Atlantic subtropical gyre. In contrast, some Holocene events include centennial-scale oscillations (~100 years) marked by MF declines in southern Iberia concomitant with SST warming in the eastern North Atlantic subtropical gyre. This climatic pattern is similar to that observed at decadal timescale under the influence of the positive mode of the North Atlantic Oscillation (NAO). We suggest, therefore, that synchronous SW Iberian dryness and SST warming at centennial timescale could be explained by atmospheric fluctuations related to NAO changes.

Impacts of the north and tropical Atlantic Ocean on the Antarctic Peninsula and sea ice

In recent decades, Antarctica has experienced pronounced climate changes. The Antarctic Peninsula exhibited the strongest warming of any region on the planet, causing rapid changes in land ice. Additionally, in contrast to the sea-ice decline over the Arctic, Antarctic sea ice has not declined, but has instead undergone a perplexing redistribution. Antarctic climate is influenced by, among other factors, changes in radiative forcing and remote Pacific climate variability, but none explains the observed Antarctic Peninsula warming or the sea-ice redistribution in austral winter. However, in the north and tropical Atlantic Ocean, the Atlantic Multidecadal Oscillation (a leading mode of sea surface temperature variability) has been overlooked in this context. Here we show that sea surface warming related to the Atlantic Multidecadal Oscillation reduces the surface pressure in the Amundsen Sea and contributes to the observed dipole-like sea-ice redistribution between the Ross and Amundsen-Bellingshausen-Weddell seas and to the Antarctic Peninsula warming. Support for these findings comes from analysis of observational and reanalysis data, and independently from both comprehensive and idealized atmospheric model simulations. We suggest that the north and tropical Atlantic is important for projections of future climate change in Antarctica, and has the potential to affect the global thermohaline circulation and sea-level change.

The Holocene onset in the southwestern South Atlantic

In this paper we present a paleoceanographic reconstruction of the southwestern South Atlantic for the past 13kyr based on faunal and isotopic analysis of planktonic foraminifera from a high-resolution core retrieved at the South Brazil Bight continental slope. Our record indicates that oceanographic changes in the southwestern South Atlantic during the onset of the Holocene were comparable in strength to those that occurred during the Younger Dryas. Full interglacial conditions started abruptly after 8.2kyrBP with a sharp change in faunal composition and surface hydrography (SST and SSS). Part of the observed events may be explained in terms of changes in thermohaline circulation while the other part suggests a dominant role of winds. Our data indicate that during the Early Holocene upwelling was significantly strengthened in the South Brazil Bight promoting high productivity and preventing the establishment of the typically interglacial menardiiform species. In general terms, oceanographic changes recorded by core KF02 occurred in synchrony with Antarctica's climate.

Reconstructing changes in Atlantic thermohaline circulation during the 20th century under two possible scenarios

Atlantic thermohaline circulation (THC) is a key component of the Earth Climate System and identification of its changes during the 20th Century is critical to the understanding of its variation characteristics and the corresponding climatic impacts. Previous researches have been inconclusive, with the results varying depending on the approach used to measure THC. The results for the two established approaches for measurement of the phenomenon (direct observation and indirect reconstruction) are contradictive (weakening and non-weakening), and their credibility needs improving. Based on the tight relationship between THC anomaly and “see-saw” intensities of Sea Surface Temperature (SST) and Surface Air Temperature (SAT), we first diagnose their quantitative relationship in the model experiments, which is corresponding to its two possible scenarios, and then reconstruct the changes of THC during the 20th Century respectively with multiple observed datasets of SST and SAT. Model results show that THC anomaly and SST/SAT “see-saw” intensities are well correlated in timescales longer than 10/40 years under scenarios of weakening/non-weakening respectively. Two kinds of reconstructions here are consistent with each other, and we propose that THC has undergone a 2-cycle oscillation with inter-decadal scale since the Industrial Revolution with a magnitude of about 1 Sv. The transformation times of decadal trend are around the mid-1910s, the 1940s, and the mid-1970s. This research further validates the main results of previous reconstructions, and points out that THC does not have a long-term weakening during the 20th Century.

Changes in upwelling mechanisms drove the evolution of marine organisms

Long-term evolution is thought to take opportunities that arise as a consequence of mass extinction (as argued, for example, by Gould, 2002) and the following biotic recovery, but there is absolutely no evidence for this being the case. However, our study shows that eutrophication by oceanic mixing also played a part in the enhancement of several evolutionary events amongst marine organisms, and these results could indicate that the rates of oceanic biodiversification may be slowed if upwelling becomes weakened by future global warming. This paper defines three distinct evolutionary events of resting spores of the marine diatom genus Chaetoceros, to reconstruct past upwelling through the analysis of several DSDP, ODP and land-based successions from the North, South and equatorial Pacific as well as the Atlantic Ocean during the past 40million years. The Atlantic Chaetoceros Explosion (ACE) event occurred across the E/O boundary in the North Atlantic, and is characterized by resting spore diversification that occurred as a consequence of the onset of upwelling following changes in thermohaline circulation through global cooling in the early Oligocene. Pacific Chaetoceros Explosion events-1 and -2 (PACE-1 and PACE-2) are characterized by relatively higher occurrences of iron input following the Himalayan uplift and aridification at 8.5Ma and ca. 2.5Ma in the North Pacific region. These events not only enhanced the diversification and increased abundance of primary producers, including that of Chaetoceros, other diatoms and seaweeds, but also stimulated the evolution of zooplankton and larger predators, such as copepods and marine mammals, which ate these phytoplankton and plants. Current thinking suggests new evolutionary niches open up after a mass extinction, but our study finds that eutrophication can also stimulate evolutionary diversification. Moreover, in the opposite fashion, our results show that as thermohaline circulation abates, global warming progresses and the ocean surface becomes warmer, many marine organisms will be affected by the environmental degradation.

High‐resolution sea surface reconstructions off Cape Hatteras over the last 10 ka

This study presents high‐resolution foraminiferal‐based sea surface temperature, sea surface salinity and upper water column stratification reconstructions off Cape Hatteras, a region sensitive to atmospheric and thermohaline circulation changes associated with the Gulf Stream. We focus on the last 10,000 years (10 ka) to study the surface hydrology changes under our current climate conditions and discuss the centennial to millennial time scale variability. We observed opposite evolutions between the conditions off Cape Hatteras and those south of Iceland, known today for the North Atlantic Oscillation pattern. We interpret the temperature and salinity changes in both regions as co‐variation of activities of the subtropical and subpolar gyres. Around 8.3 ka and 5.2–3.5 ka, positive salinity anomalies are reconstructed off Cape Hatteras. We demonstrate, for the 5.2–3.5 ka period, that the salinity increase was caused by the cessation of the low salinity surface flow coming from the north. A northward displacement of the Gulf Stream, blocking the southbound low‐salinity flow, concomitant to a reduced Meridional Overturning Circulation is the most likely scenario. Finally, wavelet transform analysis revealed a 1000‐year period pacing the δ18O signal over the early Holocene. This 1000‐year frequency band is significantly coherent with the 1000‐year frequency band of Total Solar Irradiance (TSI) between 9.5 ka and 7 ka and both signals are in phase over the rest of the studied period.

Rapid vegetation response to Lateglacial and early Holocene climatic fluctuation in the South Carpathian Mountains (Romania)

High-resolution pollen, conifer stomata and plant macrofossil analyses of two glacial lake sediments (1740 and 1990 m a.s.l.) are used to reconstruct Lateglacial (LG) and early Holocene (EH) vegetation and tree line changes in the Retezat Mountains. Our results show that during the LG, tree line was between 1750 and 1800 m a.s.l. formed by Larix decidua, Pinus mugo and Picea abies. Early LG spread to high altitudes suggests refugia of these tree species in the mountain. The Younger Dryas cooling resulted in regional steppe-tundra expansion, but tree line position and composition showed little change. The abundance of trees and shrubs decreased at 1740 m a.s.l., but species richness increased with the arrival of Pinus cembra. Our data support climate-model hindcasts for only modest decrease in accumulated growing season heat at mid-high altitudes. Regionally the pollen records suggest enhanced aridity and seasonality. In the EH, tree line reached 2000 m a.s.l. (higher than today) by ∼11,100 cal yr BP. P. mugo, P. cembra, P. abies established around the upper lake suggesting rapid increase in summer temperatures. The EH maximum of L. decidua between 11,200–10,600 cal yr BP was connected to high summer insolation. High altitude expansion of Abies alba between 10,600–10,300 cal yr BP suggested summer mean temperatures ∼2.8 °C higher then today. In comparison with other mountain sites in Europe, LG interstadial tree line was at similar altitude in the S Alps and ∼350 m higher in the Pirin Mountains. LG tree line fluctuation had similar low amplitude in the SE Alps, Retezat and Pirin Mts suggesting relatively weak influence of the North Atlantic thermohaline circulation changes on growing season temperatures.

Suborbital-scale surface and deep water records in the subtropical North Atlantic: implications on thermohaline overturn

We reconstruct millennial-scale variations in sea surface hydrography and deep water flow in the northwestern subtropical Atlantic (Ocean Drilling Program Leg 172 Sites 1056 and 1063) with a focus on Marine Isotope Stage (MIS) 9. Together with published records from this region, the new data also afford a longer-term perspective on millennial-scale changes in meridional overturning circulation spanning two full interglacial intervals (MIS 9 and 11) as well as two full glacial intervals (MIS 10 and 12). Planktic foraminiferal δ 18O values indicate relatively stable conditions during the peak warmth of MIS 9, but three large cold excursions disrupt the otherwise smooth transition toward glacial MIS 8. There is no unique response in the Site 1063 benthic foraminiferal δ 13C values that would suggest a concomitant decrease in the relative flux of NADW during these events. Similarly, there is no persistent correlation between millennial-scale variations in surface and deep water hydrography over the entire MIS 8–13 interval. While millennial-scale variations at the sea surface are most pronounced during glacial intervals (and the transitions toward glacial intervals), millennial-scale variations in the deep water hydrography tend to be largest during the warm periods. This observation supports that rapid changes in thermohaline circulation are sensitive to driving forces other than those directly related to ice sheet size. Time series analysis shows that spectral power in the benthic foraminiferal δ 13C record contains periodicities related to the second (∼10 kyr) and fourth harmonics (∼5 kyr) of precession in this record (∼20 kyr) pointing to the importance of tropical processes.

Impact of climate variability in the western Mediterranean during the last 20,000 years: oceanic and atmospheric responses

Eolian input fluctuations and paleoceanographic conditions in the western Mediterranean have been reconstructed by using a multi-proxy approach to infer climate variability for the last 20,000 yr. A sediment core from the East Alboran Sea basin provides high-resolution geochemical, mineralogical and grain size distribution records from the Last Glacial Maximum onward. The accurate chronology obtained, closely related with the North Greenland Ice core Project, has provided a detailed paleoclimate reconstruction at the centennial–millennial time scale. Mineral and chemical compositions as well as sediment grain size distribution have been used to establish both fluvial and eolian input variations, deep-water current intensity, and ventilation and productivity conditions at the time of deposition. During the cold period Greenland Stadial-2a, the analyzed proxies evidence significant climate oscillations. Redox proxies, showing marked Mn/Al and Fe/Al peaks, as well as granulometric parameters, support major changes in thermohaline circulation, with active circulation triggered by the Western Mediterranean Deep Water formation. Ba/Al ratios and the presence of authigenic barite point to high productivity during the Last Heinrich event. At this time, Zr/Al, Si/Al and Ti/Al ratios as well as quartz and palygorskite contents also corroborate dryer conditions in the Alboran borderlands, with a major reorganization of atmospheric circulation at the beginning of the deglaciation and during the Last Heinrich event. In contrast, a decreasing trend in the Zr/Al ratio and increasing Mg/Al, K/Al and illite + chlorite/kaolinite ratios would indicate enhanced river runoff and supporting lower eolian dust input and wetter periods during the Bölling–Alleröd. Short-term and abrupt climate oscillations such as the Intra-Alleröd Cold Period are likewise reflected by minor yet conspicuous changes in eolian proxies. During the Younger Dryas, we distinguish an initial cold and dry phase, with major westerlies intensity, thermohaline reactivation, and high productivity levels, and a second wetter phase characterized by sporadic rainfalls over poorly vegetated surrounding continental areas. Then, during the Holocene, alternating phases of major humidity and aridity were recorded by major fluctuations in detrital input. These are also correlated with recurrent forest cover declines reported from pollen records, with a noteworthy early decline in the African Humid Period. During the Holocene, an initial demise of the Last Organic-Rich Layer is also evidenced, tentatively associated with aspiration forces promoted by intense thermohaline circulation through the Gibraltar Strait.