Ocean Surface Conditions Research Articles

The impact of changing surface ocean conditions on the dissolution of aerosol iron

AbstractThe proportion of aerosol iron (Fe) that dissolves in seawater varies greatly and is dependent on aerosol composition and the physicochemical conditions of seawater, which may change depending on location or be altered by global environmental change. Aerosol and surface seawater samples were collected in the Sargasso Sea and used to investigate the impact of these changing conditions on aerosol Fe dissolution in seawater. Our data show that seawater temperature, pH, and oxygen concentration, within the range of current and projected future values, had no significant effect on the dissolution of aerosol Fe. However, the source and composition of aerosols had the most significant effect on the aerosol Fe solubility, with the most anthropogenically influenced samples having the highest fractional solubility (up to 3.2%). The impact of ocean warming and acidification on aerosol Fe dissolution is therefore unlikely to be as important as changes in land usage and fossil fuel combustion. Our experimental results also reveal important changes in the size distribution of soluble aerosol Fe in solution, depending on the chemical conditions of seawater. Under typical conditions, the majority (77–100%) of Fe released from aerosols into ambient seawater existed in the colloidal (0.02–0.4 µm) size fraction. However, in the presence of a sufficient concentration of strong Fe‐binding organic ligands (10 nM) most of the aerosol‐derived colloidal Fe was converted to soluble Fe (<0.02 µm). This finding highlights the potential importance of organic ligands in retaining aerosol Fe in a biologically available form in the surface ocean.

Changes to the Canadian Arctic Archipelago Sea Ice and Freshwater Fluxes in the Twenty-First Century under the Intergovernmental Panel on Climate Change A1B Climate Scenario

A coupled ocean and sea-ice pan-Arctic model forced by the Intergovernmental Panel on Climate Change A1B climate scenario is used to study the evolution of ice and ocean surface conditions within the Canadian Arctic Archipelago (CAA) during the twenty-first century. A summer ice-free CAA is likely by the end of our simulation. Sea ice undergoes significant changes from the mid-2020s to the mid-2060s in both concentration and thickness. The simulation shows a shrinking of 65% and a thinning of 75% in summer over the 40 years, resulting in a partially open Northwest Passage by the 2050s. However, ice in central Parry Channel might increase due to a decrease in export from April to June, linked to a reduced cross-channel sea surface height (SSH) gradient, before melting thermodynamically. On a larger scale, the central CAA throughflow will experience a significant decrease in both volume and freshwater transport after 2020, which is related to the change in the SSH difference between the two ends of Parry Channel, particularly the lifting of SSH in Baffin Bay. With a lower albedo, a warmer ocean is simulated, particularly in summer. The sea surface salinity within the CAA demonstrates a strong decadal oscillation without a clear trend over the entire simulation. A north–south pattern, separated by Parry Channel, is also found in the changes of ocean temperature and salinity fields due to different ice conditions.

Preliminary remote sensing observation of sea surface temperature increase during Ulva prolifera blooms

A massive bloom of green macroalgae Ulva prolifera occurred in June 2008 in the Yellow Sea, resulting in perhaps the largest “green tide” event in the history of bloom research. The covered area is about 1,200 km2 and the impacted area reached 40,000 km2 on 31 May. This also occurred in the following two years, 2009 and 2010. We analyzed the satellite data from MODIS and the results showed that sea surface temperature increased 1–3°C in U. prolifera bloom locations. The macroalgae in the water may have been the cause of the increase of sea surface temperatures. Rapid increases of U. prolifera biomass during the blooms may have increased radiation absorption of the water, and thus enhanced the rate of heating at the sea surface. The present study represents a preliminary observation, which is an important step for understanding influences of macroalgae on ocean surface conditions.

Hydrogen and oxygen isotopic compositions of water vapor over the Bay of Bengal during monsoon

AbstractδDv and δ18Ov of ~70 water vapor samples collected at 6 and 25 m above sea level over the Bay of Bengal (BoB) during July–August 2012 are reported. This helps characterize the isotopic signature of monsoon vapor. No significant vertical variation is observed in δDv, δ18Ov, or deuterium excess (defined as δD–8δ18O); δDv and δ18Ov are significantly correlated (r = 0.92) at each height; the deuterium excess values do not, because the variation of δDv and δ18Ov relative to their uncertainties is much larger than that of the latter. The temporal variations of δDv and δ18Ov correlate well with air temperature rather than sea surface temperature. The control of normalized humidity on deuterium excess is less prominent. While the distribution of water vapor isotopologues over the BoB is primarily determined by the ocean surface conditions, they are significantly altered by laterally advected vapor from rain en route during the monsoon.

Deep ocean communities impacted by changing climate over 24 y in the abyssal northeast Pacific Ocean

The deep ocean, covering a vast expanse of the globe, relies almost exclusively on a food supply originating from primary production in surface waters. With well-documented warming of oceanic surface waters and conflicting reports of increasing and decreasing primary production trends, questions persist about how such changes impact deep ocean communities. A 24-y time-series study of sinking particulate organic carbon (food) supply and its utilization by the benthic community was conducted in the abyssal northeast Pacific (~4,000-m depth). Here we show that previous findings of food deficits are now punctuated by large episodic surpluses of particulate organic carbon reaching the sea floor, which meet utilization. Changing surface ocean conditions are translated to the deep ocean, where decadal peaks in supply, remineralization, and sequestration of organic carbon have broad implications for global carbon budget projections.

Mid‐Holocene Asian summer climate and its responses to cold ocean surface simulated in the PMIP2 OAGCMs experiments

AbstractIn this study, the outputs from four Ocean‐Atmosphere Coupled General Circulation Model experiments within the Paleoclimate Modelling Intercomparison Project phase 2 and four sets of Atmospheric General Circulation Model experiments were used to analyze the Asian summer climate during the mid‐Holocene (6 ka, about 6000 years ago). Additionally, the role of the orbital forcings and the effects of a cold ocean surface for the 6 ka were investigated by comparing the Atmospheric General Circulation Model simulations forced by different combinations of forcing parameters. The results indicated that in the 6 ka summer, the orbital forcings were the prime drivers of the increased temperature and precipitation and the strengthened summer monsoon over the Asian continent. On the other hand, these different orbital forcings also resulted in a colder Indian Ocean‐northwestern Pacific during this period. Our results suggested that this cold ocean surface could reduce the warming amplitude and precipitation enhancement over the Asian monsoon area in the 6 ka summer. The changes caused by the different ocean surface conditions were comparable with simulated 6 ka climate changes. The cold ocean surface also suppressed the Asian summer monsoon circulations. Therefore, the influences from anomalous ocean surface conditions played an important role on regulating the Asian summer climate during the 6 ka. In addition, it was found that the summer climate in the South Asian monsoon area was more sensitive to the changes in the orbital forcings and ocean surface conditions than that in the East Asian monsoon area.

Mid-Holocene climate in New Caledonia (southwest Pacific): coral and PMIP models monthly resolved results

The mid-Holocene climate is characterized by an insolation seasonality decrease in the Southern Hemisphere but measurements of its actual impact on monthly resolved sea surface temperature (SST) in the southwest Pacific region are still insufficient. A New Caledonian 5.5 ka cal BP coral provides a 20-year-long seasonally-resolved record of ocean surface conditions as inferred from coral Sr/Ca and Ba/Ca. Results were compared to monthly series of Paleoclimate Modeling Intercomparison Project phase 2 (PMIP2) model simulations. Anomalous stable isotope and U/Ca values are observed in a restricted area of the skeleton related to dissolution features. The mid-Holocene SST seasonal amplitude mean and variability are higher than presently in New Caledonia, the increased seasonal amplitude in the mid-Holocene being most probably due to the occurrence of colder winters. Other southwest Pacific mid-Holocene coral data showed also such an increased seasonal amplitude. This could mean that the South Pacific Convergence Zone (SPCZ) was weaker or reached locations more northerly than at present, which could fit with northward shifts of the inter-tropical convergence zone (ITCZ) during South Hemisphere winter. Inversely, strong rainfalls during the summer, deduced from the Ba/Ca signal and reconstructed sea surface salinity (SSS), were interpreted as reflecting pronounced southwestward shifts of the SPCZ in summer, as those occurring today during La Niña events. None of the six PMIP2 models used reproduce the proxy-based mid-Holocene increase of SST seasonal amplitude. Model maps show a less intense SPCZ in winter that would be consistent with higher SST seasonal amplitude. Finally, we stress the need for more seasonally-resolved data to validate this enhanced mid-Holocene SST seasonal amplitude in the southwest Pacific region and to better understand the underlying mechanisms.

Reconstructions of surface ocean conditions from the northeast Atlantic and Nordic seas during the last millennium

We undertake the first comprehensive effort to integrate North Atlantic marine climate records for the last millennium, highlighting some key components common within this system at a range of temporal and spatial scales. In such an approach, careful consideration needs to be given to the complexities inherent to the marine system. Composites therefore need to be hydrographically constrained and sensitive to both surface water mass variability and three-dimensional ocean dynamics. This study focuses on the northeast (NE) North Atlantic Ocean, particularly sites influenced by the North Atlantic Current. A composite plus regression approach is used to create an inter-regional NE North Atlantic reconstruction of sea surface temperature (SST) for the last 1000 years. We highlight the loss of spatial information associated with large-scale composite reconstructions of the marine environment. Regional reconstructions of SSTs off the Norwegian and Icelandic margins are presented, along with a larger-scale reconstruction spanning the NE North Atlantic. The latter indicates that the ‘Medieval Climate Anomaly’ warming was most pronounced before ad 1200, with a long-term cooling trend apparent after ad 1250. This trend persisted until the early 20th century, while in recent decades temperatures have been similar to those inferred for the ‘Medieval Climate Anomaly’. The reconstructions are consistent with other independent records of sea-surface and surface air temperatures from the region, indicating that they are adequately capturing the climate dynamics of the last millennium. Consequently, this method could potentially be used to develop large-scale reconstructions of SSTs for other hydrographically constrained regions.

Modeling insights into deuterium excess as an indicator of water vapor source conditions

Deuterium excess (d) is interpreted in conventional paleoclimate reconstructions as a tracer of oceanic source region conditions, such as temperature, where precipitation originates. Previous studies have adopted coisotopic approaches (using both δ18O and d) to estimate past changes in both site and oceanic source temperatures for ice core sites using empirical relationships derived from conceptual distillation models, particularly Mixed Cloud Isotopic Models (MCIMs). However, the relationship between d and oceanic surface conditions remains unclear in past contexts. We investigate this climate‐isotope relationship for sites in Greenland and Antarctica using multiple simulations of the water isotope‐enabled Goddard Institute for Space Studies ModelE‐R general circulation model and apply a novel suite of model vapor source distribution (VSD) tracers to assess d as a proxy for source temperature variability under a range of climatic conditions. Simulated average source temperatures determined by the VSDs are compared to synthetic source temperature estimates calculated using MCIM equations linking d to source region conditions. We show that although deuterium excess is generally a faithful tracer of source temperatures as estimated by the MCIM approach, large discrepancies in the isotope‐climate relationship occur around Greenland during the Last Glacial Maximum simulation, when precipitation seasonality and moisture source regions were notably different from the present. This identified sensitivity in d as a source temperature proxy suggests that quantitative climate reconstructions from deuterium excess should be treated with caution for some sites when boundary conditions are significantly different from the present day. Also, the exclusion of the influence of humidity and other evaporative source changes in MCIM regressions may be a limitation of quantifying source temperature fluctuations from deuterium excess in some instances.

Multiproxy fossil comparison reveals contrasting surface ocean conditions in the western Iceland Sea for the last two interglacials

Dinoflagellate cyst (dinocyst), coccolith and planktic foraminiferal assemblages from a core in the western Iceland Sea were used to reconstruct and compare the surface ocean developments of the Holocene and the Last Interglacial (Marine Isotopic Stage or MIS 5e). While increasing subpolar planktic foraminifers from ~10ka indicate subsurface warming peaking around 7.5ka, the dinocyst data suggest that the uppermost ocean remained dominated by cold waters until ~6.5ka. A reduced advection of cold polar waters through the East Greenland/East Icelandic Current thereafter entailed warmest and most saline Holocene conditions between 6.5 and 5ka, in turn followed by a general cooling trend. By contrast, both planktic foraminifers and dinocysts show an increased presence of Atlantic (-type) waters from ~122ka onward resulting in a MIS 5e thermal optimum around 120.5ka. Nonetheless, occasional freshwater input from melting drift ice created stratified but also seasonally variable conditions during this first half of MIS 5e. This stratification signature disappeared at ~120ka when a marked repositioning of the oceanographic fronts occurred. Slightly colder conditions are indicated by both phyto- and zooplankton from there on until the end of MIS 5e around ~117ka. A late MIS 5e cooling at the Iceland Plateau is opposite to a late MIS 5e optimum observed in the eastern Nordic seas. This regional difference is likely explained by various feedback mechanisms following the major reorganisation of the oceanic fronts at ~120ka. The Holocene and MIS 5e interglacial variability is not reflected in the (quasi-monospecific) coccolith assemblages and illustrates the low sensitivity of living coccolithophore communities to subtle temperature changes in the low-temperature regions of the Nordic seas. Overall, quite different surface water properties appear to have characterised both interglacial intervals, with a higher share of warm, Atlantic elements in the planktic communities during MIS 5e. This suggests a higher contribution of Atlantic waters in the southwestern Nordic seas probably due to a more northward expansion of the Irminger Current under weakened polar East Greenland/East Icelandic currents. Such a reduced influence of polar waters in the southwestern Nordic seas may thus explain other evidence for relatively warm conditions in MIS 5e all around southern Greenland.

Diatom evidence for the onset of Pliocene cooling from AND-1B, McMurdo Sound, Antarctica

The late Pliocene, ~3.3–3.0Ma, is the most recent interval of sustained global warmth in the geologic past. This window is the focus of climate reconstruction efforts by the U.S. Geological Survey's Pliocene Research, Interpretation, and Synoptic Mapping (PRISM) Data/Model Cooperative, and may provide a useful climate analog for the coming century. Reconstructions of past surface ocean conditions proximal to the Antarctic continent are essential to understanding the sensitivity of the cryosphere to this key interval in Earth's climate evolution. An exceptional marine sediment core collected from the southwestern Ross Sea (78° S), Antarctica, during ANDRILL's McMurdo Ice Shelf Project preserves evidence of dramatic fluctuations between grounded ice and productive, open ocean conditions during the late Pliocene, reflecting orbitally-paced glacial/interglacial cycling. In this near-shore record, diatom-rich sediments are recovered from interglacial intervals; two of these diatomites, from ~3.2Ma and 3.03Ma, are within the PRISM chronologic window. The diatom assemblages identified in PRISM-age late Pliocene diatom-rich sediments are distinct from those in mid-Pliocene and later Pliocene/Pleistocene intervals recovered from AND-1B, and comprise both extant taxa with well-constrained ecological preferences and a diverse extinct flora, some members of which are previously undescribed from Antarctic sediments. Both units are dominated by Chaetoceros resting spores, an indicator of high productivity and stratification that is present at much lower abundance in materials both older and younger than the PRISM-age sediments. Newly described species of the genus Fragilariopsis, which first appear in the AND-1B record at 3.2Ma, are the most abundant extinct members of the PRISM-age assemblages. Other extant species with established environmental affinities, such as Fragilariopsis sublinearis, F. curta, Stellarima microtrias, and Thalassiothrix antarctica, are present at lower abundances. Environmental inferences drawn from extant diatom assemblages are in good agreement with those from Chaetoceros resting spores and the Fragilariopsis radiation. All three lines of evidence indicate the onset of late Pliocene cooling in the Ross Sea near-shore environment at 3.2Ma, with intensification and possible regional persistence of summer sea ice by 3.03Ma. An important implication of this research is the indication that the Ross Ice Shelf fluctuated dramatically on orbital timescales at a time when nearshore Antarctic conditions were only modestly warmer than present.

Evaluating Southern Ocean sea-ice for the Last Glacial Maximum and pre-industrial climates: PMIP-2 models and data evidence

Sea ice is recognized as an important factor controlling climate conditions at high latitudes. It has a major impact in controlling the ocean to atmosphere exchanges (e.g. for CO2) as well as surface oceanic conditions and deep water formation in the high latitude oceans. Being at the interface of two major components of the climate system, sea ice is useful to evaluate and intercompare different climate models. We here analyze the results of eight coupled climate models (from which seven are from the PMIP-2 database) against available reconstructions for winter and summer sea ice extent around Antarctica. To evaluate the ability of models in representing different climate states, we compare their results for both the pre-industrial and the Last Glacial Maximum (LGM). We find that models generally have identical seasonal ranges for the two climate states analyzed, which is not supported by data inferences. Indeed, data indicate that the LGM seasonality was enhanced compared to the pre-industrial. The shape of the sea ice distribution is also different in models (circular around the Antarctic continent) and in reconstructions (oval-shaped distribution; similar to modern sea ice distribution). We show that models underestimating the sea ice extent for the pre-industrial are also underestimating the Last Glacial Maximum extent. Our results thus point to difficulties for climate models in representing the sea-ice dynamics in the Southern Ocean.

Ocean Acidification Refugia of the Florida Reef Tract

Ocean acidification (OA) is expected to reduce the calcification rates of marine organisms, yet we have little understanding of how OA will manifest within dynamic, real-world systems. Natural CO2, alkalinity, and salinity gradients can significantly alter local carbonate chemistry, and thereby create a range of susceptibility for different ecosystems to OA. As such, there is a need to characterize this natural variability of seawater carbonate chemistry, especially within coastal ecosystems. Since 2009, carbonate chemistry data have been collected on the Florida Reef Tract (FRT). During periods of heightened productivity, there is a net uptake of total CO2 (TCO2) which increases aragonite saturation state (Ωarag) values on inshore patch reefs of the upper FRT. These waters can exhibit greater Ωarag than what has been modeled for the tropical surface ocean during preindustrial times, with mean (± std. error) Ωarag-values in spring = 4.69 (±0.101). Conversely, Ωarag-values on offshore reefs generally represent oceanic carbonate chemistries consistent with present day tropical surface ocean conditions. This gradient is opposite from what has been reported for other reef environments. We hypothesize this pattern is caused by the photosynthetic uptake of TCO2 mainly by seagrasses and, to a lesser extent, macroalgae in the inshore waters of the FRT. These inshore reef habitats are therefore potential acidification refugia that are defined not only in a spatial sense, but also in time; coinciding with seasonal productivity dynamics. Coral reefs located within or immediately downstream of seagrass beds may find refuge from OA.

Tropical-temperate interactions over southern Africa simulated by a regional climate model

The Weather Research and Forecasting model (WRF) forced by ERA40 re-analyses, is used to examine, at regional scale, the role of key features of the local atmospheric circulation on the origin and development of Tropical Temperate Troughs (TTTs) representing a major contribution to South African rainfall during austral summer. A cluster analysis applied on 1971–2000 ERA40 and WRF simulated daily outgoing longwave radiation reveals for the November–February season three coherent regimes characteristic of TTTs over the region. Analyses of WRF simulated TTTs suggest that their occurrence is primarily linked with mid-latitude westerly waves and their phasing. Ensemble experiments designed for the case of austral summer 1996/1997 allow to examine the reproducibility of TTT events. The results obtained illustrate the importance of westerly waves phasing regarding the persistence of rain-producing continental TTT events. Moreover, oceanic surface conditions prevailing over the Agulhas current regions of the South West Indian Ocean (SWIO) are also found to influence TTT persistence for regional experiments with an oceanic mixed layer, warmer sea surface temperatures being associated with increased moisture advection from the SWIO where latent heat release is enhanced, favoring baroclinic instability and thus sustaining convection activity locally.

Persisting maximum Agulhas leakage during MIS 14 indicated by massive Ethmodiscus oozes in the subtropical South Atlantic

The glacial marine isotope stage 14 (MIS 14) appears in many climate records as an unusually warm glacial. During this period an almost monospecific, up to 1.5 m thick, laminated layer of the giant diatom Ethmodiscus rex has been deposited below the South Atlantic Subtropical Gyre. This oligotrophic region is today less favorable for diatom growth with sediments typically consisting of calcareous nannofossil oozes. We have reconstructed temperatures and the stable oxygen isotopic compositions of sea surface and thermocline water (δ18Ow) from planktonic foraminiferal (Globigerinoides ruber and Globorotalia inflata) Mg/Ca and stable oxygen isotopes to test whether perturbations in surface ocean conditions contributed to the deposition of the diatom layer at ∼530 kyr B.P. Temperatures and δ18Ow values reconstructed from this diatom ooze interval are highly variable, with maxima similar to interglacial values. Since the area of the Ethmodiscus oozes resembles the region where Agulhas rings are present, we interpret these hydrographic changes to reflect the varying influence of warm and saline water of Indian Ocean origin that entered the Subtropical Gyre trapped in Agulhas rings. The formation of the Ethmodiscus oozes is associated with a period of maximum Agulhas leakage and a maximum frequency of Agulhas ring formation caused by a termination‐type position of the Subtropical Front during the unusual warm MIS 14. The input of silica through the Agulhas rings enabled the shift in primary production from calcareous nannoplankton to diatoms, leading to the deposition of the massive diatom oozes.

The Effects of Natural Iron Fertilisation on Deep-Sea Ecology: The Crozet Plateau, Southern Indian Ocean

The addition of iron to high-nutrient low-chlorophyll (HNLC) oceanic waters stimulates phytoplankton, leading to greater primary production. Large-scale artificial ocean iron fertilization (OIF) has been proposed as a means of mitigating anthropogenic atmospheric CO2, but its impacts on ocean ecosystems below the photic zone are unknown. Natural OIF, through the addition of iron leached from volcanic islands, has been shown to enhance primary productivity and carbon export and so can be used to study the effects of OIF on life in the ocean. We compared two closely-located deep-sea sites (∼400 km apart and both at ∼4200 m water depth) to the East (naturally iron fertilized; +Fe) and South (HNLC) of the Crozet Islands in the southern Indian Ocean. Our results suggest that long-term geo-engineering of surface oceanic waters via artificial OIF would lead to significant changes in deep-sea ecosystems. We found that the +Fe area had greater supplies of organic matter inputs to the seafloor, including polyunsaturated fatty acid and carotenoid nutrients. The +Fe site also had greater densities and biomasses of large deep-sea animals with lower levels of evenness in community structuring. The species composition was also very different, with the +Fe site showing similarities to eutrophic sites in other ocean basins. Moreover, major differences occurred in the taxa at the +Fe and HNLC sites revealing the crucial role that surface oceanic conditions play in changing and structuring deep-sea benthic communities.

A new mechanism for ocean–atmosphere coupling in midlatitudes

AbstractThe role of moist convection in ‘transferring’ upward surface ocean conditions throughout the troposphere is studied using reanalysis data for the extratropical Northern and Southern Hemispheres in winter. It is found that conditions for the development of a convective air column from the sea surface to the tropopause are met frequently over all major western boundary currents and their extension in the oceanic interior (sometimes for as much as 50% of the time). These frequent occurrences are shown to be jointly controlled by oceanic advection of warm waters and, on the atmospheric side, the downward displacement of the tropopause associated with synoptic weather systems.Based on these results, it is proposed that the oceans can influence the atmosphere directly through convection in midlatitudes, as is commonly thought to occur in the Tropics. Analysis of the Richardson numberRifound at low levels suggests that moist symmetric instability (0< Ri≤ 1) is a key process involved in linking surface ocean temperatures to atmospheric lapse rates, in addition to standard upright convection. These lowRiprocesses are not currently parametrized in climate models, which raises the possibility that the extratropical oceanic influence on climate might be underestimated in the current generation of models. Copyright © 2011 Royal Meteorological Society

Coral windows onto seasonal climate variability in the northern Caribbean since 1479

Mean surface ocean conditions in the Caribbean were up to ∼2°C cooler than today at times during the Little Ice Age. The seasonal context for such mean state changes is important for determining the mechanisms involved. We reconstructed surface ocean conditions in southwest Puerto Rico at approximately monthly resolution over eight 4–12 year periods during the last ∼520 years to test if the seasonal cycles of temperature or salinity varied with mean state. We carried out paired analyses of Sr/Ca and δ18O for two coral cores. The δ18O data contained clear annual cycles and were significantly correlated to temperature during the 20th century calibration periods (1993–2004 and 1902–1912, r = 0.73). The Sr/Ca data contained high‐frequency noise that obscured the seasonal cycles, although the centennial variability matched that of the coral δ18O, indicating a common forcing that is likely temperature. The seasonal coral δ18O amplitude averaged 0.60 ± 0.17‰, with none of the periods significantly different from the most recent. The simplest explanation is that the amplitudes of seasonal seawater δ18O and temperature variations were not different from today. Previous work in the southern Caribbean indicates that the Intertropical Convergence Zone was shifted southward or weaker during the Little Ice Age, and we speculate about how this could occur with no apparent affect on seasonality in the northern Caribbean.

Atlantic overturning circulation and Agulhas leakage influences on southeast Atlantic upper ocean hydrography during marine isotope stage 11

Climate dynamics during the marine isotope stage (MIS) 11 interglacial may provide information about how the climate system will evolve under the conditions of low-amplitude orbital forcing that are also found during the late Holocene. New stable isotope and alkenone data are presented from southeast Atlantic Ocean Drilling Program Site 1085, providing detailed information on interglacial climate evolution and the impacts of Atlantic meridional overturning circulation (MOC) and Agulhas leakage on the regional upper ocean hydrography. The data suggest that although warm surface ocean conditions were maintained at approximate Holocene levels for 40,000 years during MIS 11, subsurface temperature and salinity recorded by deeper-dwelling planktonic foraminifera species were maintained at their highest values for only 7000–8000 years. Surface water temperature and salinity data suggest that the interocean exchange of warm, salty waters into the southeast Atlantic Ocean was directly related to changes in the activity of the MOC during the study interval. Specifically, transient regional warming events during periods of weakened overturning circulation may have been amplified by the continuous interocean exchange of warm, salty Indian Ocean waters that primed the MOC for abrupt resumptions into a vigorous mode of operation. Conversely, a peak in interocean exchange at the end of the MIS 11 interglacial optimum may reflect enhanced trade wind forcing of surface waters whose export to the North Atlantic Ocean could have contributed to renewed ice sheet buildup during the MIS 11 to 10 glacial inception.

Settling silicoflagellates at the southwestern region of the Gulf of California.

Silicoflagellates can be used, after their sedimentation, as a proxy of prevailing ocean surface conditions, however it is essential to identify and document the species inhabiting the study area. This includes a visual and descriptive record of their morphological variability which was accomplished by continuously monitoring the settling silicoflagellates in the Alfonso Basin (Bahia de La Paz) from January 2002 to December 2003.Thirteen taxa distributed in 4 genera: Corbisema (1 species), Dictyocha (10 taxa),Octactis (1 species) and Distephanus (1 species) were identified. Highest number of taxa (10 and 11respectively) was observed during the summer and autumn seasons of both years. Octactis pulchra was the only species observed in the 65 samples analyzed. Certain taxa such as Dictyocha calida, D. fibula var robusta, D. epiodon and O. pulchra showed variations in their skeletons which suggest a group response to varying oceanographic conditions, making it imperative to document this variability.