ALBERT

Description: Living (stained) benthic foraminiferal faunas were analyzed in 51 surficial sediment samples taken from the northern Gulf of Cadiz continental shelf between the mouths of the Guadiana and Guadalquivir rivers. The distribution and abundance of 26 species with relative abundance 〉5% were related to water depth, sediment type, river discharge, water temperature, salinity, turbidity, and primary productivity. Hierarchical classification using R- and Q-mode cluster analyses, and individual distributions, allowed these taxa to be categorized into four general groups. Group 1 is represented by Bolivina ordinaria and Hopkinsina atlantica. These species are linked to river discharge, and are associated with muddy sediments. They are considered the most opportunistic species in the assemblage, and are associated with the limits of productivity. Group 2, contains Ammonia beccarii, Eggerelloides scaber, Elphidium gerthi, Quinqueloculina laevigata, and Q. stelligera, and is characteristic of shallow water depths. Its species are associated with different sediment types and their distribution is influenced by river discharge. Group 3 is dominated by Bulimina aculeata, B. elongata, Elphidium excavatum, E. cuvillieri, Epistominella vitrea, and Rectuvigerina phlegeri. These species display the highest abundances between 30–100 m water depths. They prevail in muds associated with weak hydrodynamics, low oxygenation, and high organic matter. Group 4 includes species that are more abundant in deep stations and is divided into two subgroups. Subgroup 4A includes Bolivina catanensis, B. italica, B. striatula, Cassidulina laevigata, Stainforthia sp., Nouria polymorphinoides, and Nouria sp., with higher abundances in one or two samples. Subgroup 4B, with Brizalina dilatata, B. spathulata, Bulimina marginata, Nonionella iridea, N. stella, and N. turgida, has patchy distribution. Higher abundances of Nonionella species off the Guadiana River are related to low temperature and salinity.

Description: Habitat patterns of subtropical and tropical planktic foraminifers in the Caribbean Sea were obtained from plankton samples collected in spring 2009 and 2013. The spatial distribution in surface waters (3.5 m water depth) and depth habitat patterns (surface to 400 m) of 33 species were compared with prevailing water-mass conditions (temperature, salinity, and chlorophyll-a concentration) and planktic foraminiferal test assemblages in surface sediments. Distribution patterns indicate a significant relationship with seawater temperature and trophic conditions. A reduction in standing stocks was observed close to the Orinoco River plume and in the Gulf of Paria, associated with high turbidity and concomitant low surface-water salinity. In contrast, a transient mesoscale patch of high chlorophyll concentration in the eastern Caribbean Sea was associated with higher standing stocks in near surface waters, including high abundances of Globigerinita glutinata and Neogloboquadrina dutertrei. Globorotalia truncatulinoides mainly lives close to the seasonal pycnocline and can be linked to winter conditions indicated by lower sea-surface temperatures (SSTs) of ∼20°C. Globigerinoides sacculifer and Globoturborotalita rubescens were associated with oligotrophic conditions in the pelagic Caribbean Sea during early spring and showed a synodic lunar reproduction cycle. The live assemblages in the water column from 2009 and 2013 were similar to those reported in earlier studies from the 1960s and 1990s and to assemblages of tests in the surface sediments. Minor differences in faunal proportions were attributed to seasonal variability and environmental differences at the local scale. An exception was the low relative abundance of Globigerinoides ruber in the Caribbean Sea in 2009 compared to surface sediment samples and plankton net samples collected in the 1960s and 1990s. Decreasing abundance of Gs. ruber white in the Caribbean Sea may be associated with increasing SSTs over past decades and changes in nutrient flux and primary production.

Description: The Atlantic‐Mediterranean exchange of water at Gibraltar represents a significant heat and freshwater sink for the North Atlantic and is a major control on the heat, salt and freshwater budgets of the Mediterranean Sea. Consequently, an understanding of the response of the exchange system to external changes is vital to a full comprehension of the hydrographic responses in both ocean basins. Here, we use a synthesis of empirical (oxygen isotope, planktonic foraminiferal assemblage) and modeling (analytical and general circulation) approaches to investigate the response of the Gibraltar Exchange system to Atlantic freshening during Heinrich Stadials (HSs). HSs display relatively flat W–E surface hydrographic gradients more comparable to the Late Holocene than the Last Glacial Maximum. This is significant, as it implies a similar state of surface circulation during these periods and a different state during the Last Glacial Maximum. During HS1, the gradient may have collapsed altogether, implying very strong water column stratification and a single thermal and d18Owater condition in surface water extending from southern Portugal to the eastern Alboran Sea. Together, these observations imply that inflow of Atlantic water into the Mediterranean was significantly increased during HS periods compared to background glacial conditions. Modeling efforts confirm that this is a predictable consequence of freshening North Atlantic surface water with iceberg meltwater and indicate that the enhanced exchange condition would last until the cessation of anomalous freshwater supply into to the northern North Atlantic. The close coupling of dynamics at Gibraltar Exchange with the Atlantic freshwater system provides an explanation for observations of increased Mediterranean Outflow activity during HS periods and also during the last deglaciation. This coupling is also significant to global ocean dynamics, as it causes density enhancement of the Atlantic water column via the Gibraltar Exchange to be inversely related to North Atlantic surface salinity. Consequently, Mediterranean enhancement of the Atlantic Meridional Overturning Circulation will be greatest when the overturning itself is at its weakest, a potentially critical negative feedback to Atlantic buoyancy change during times of ice sheet collapse.

Description: The pore-densities (PD) in the tests of 232 specimens of the shallow infaunal foraminiferal species Bolivina spissa from eight locations off the Peruvian continental margin were investigated and compared to different environmental factors as water-depth, temperature, bottom-water oxygen ([O2]BW) and nitrate concentrations ([NO3–]BW). There is a negative exponential PD-[O2]BW correlation, but at oxygen-concentrations 〉10 µmol/l PD approaches a constant value without any further correlation to [O2]BW. The PD-[NO3–]BW relationship is better constrained than that for PD-[O2]BW. We hypothesize that the pores in the tests of B. spissa largely reflect the intracellular nitrate, and to a smaller extent the oxygen respiration. We also compared PD and porosity (P) of two single B.spissa and B.seminuda specimens from the same habitat. The comparison showed that P is significantly higher in B.seminuda than in B. spissa indicating that B.seminuda is much better adapted to strong oxygen-depleted habitats than B.spissa.

Description: Deep marine currents are strongly influenced by climatic changes. They also deposit, rework, and sort sediment, and can generate kilometer-scale sedimentary bodies (drifts). These drifts are made of thoroughly bioturbated, stacked sedimentary sequences called contourites [Gonthier et al., 1984]. As a consequence, change in the direction or intensity of currents can be recorded in the sediments

Description: A benthic isotope record has been measured for core SO75-26KL from the upper Portuguese margin (1099 m water depth) to monitor the response of thermohaline overturn in the North Atlantic during Heinrich events. Evaluating benthic δ18O in TS diagrams in conjunction with equilibrium δc fractionation implies that advection of Mediterranean outflow water (MOW) to the upper Portuguese margin was significantly reduced during the last glacial (〈 15% compared to 30% today). The benthic isotope record along core SO75-26KL therefore primarily monitors variability of glacial North Atlantic conveyor circulation. The 14C-accelerator mass spectrometry ages of 13.54±.07 and 20.46±.12 ka for two ice-rafted detritus (IRD) layers in the upper core section and an interpolated age of 36.1 ka for a third IRD layer deeper in the core are in the range of published 14C ages for Heinrich events H1, H2, and H4. Marked depletion of benthic δ13C by 0.7–1.1‰ during the Heinrich events suggests reduced thermohaline overturn in the North Atlantic during these events. Close similarity between meltwater patterns (inferred from planktonic δ18O) at Site 609 and ventilation patterns (inferred from benthic δ13C) in core SO75-26KL implies coupling between thermohaline overturn and surface forcing, as is also suggested by ocean circulation models. Benthic δ13C starts to decrease 1.5–2.5 kyr before Heinrich events Hl and H4, fully increased values are reached 1.5–3 kyr after the events, indicating a successive slowdown of thermohaline circulation well before the events and resumption of the conveyor's full strength well after the events. Benthic δ13C changes in the course of the Heinrich events show subtle maxima and minima suggesting oscillatory behavior of thermohaline circulation, a distinct feature of thermohaline instability in numerical models. Inferrred gradual spin-up of thermohaline circulation after Hl and H4 is in contrast to abrupt wanning in the North Atlantic region that is indicated by sudden increases in Greenland ice core δ18O and in marine faunal records from the northern North Atlantic. From this we infer that thermohaline circulation can explain only in part the rapid climatic oscillations seen in glacial sections of the Greenland ice core record.

Description: Modeling and proxy studies indicate that a reduction of Atlantic Meridional Overturning Circulation (AMOC) strength profoundly impacts temperatures and salinities in the (sub)tropical Atlantic, especially on subsurface levels. While previous studies focused on prominent periods of AMOC reduction during the last deglaciation, we aim to test whether similar reconfigurations of the subtropical hydrography occurred during the moderate climatic alterations punctuating the last interglacial Marine Isotope Stage (MIS) 5. Here, we present temperature and salinity records from a Florida Straits core by combining d18O and Mg/Ca analyses on surface (Globigerinoides ruber, white) and deep‐dwelling (Globorotalia crassaformis) foraminifera covering MIS 5 in high resolution. The data reveal increasing salinities at intermediate depths during interglacial cooling episodes, decoupled from relatively stable surface conditions. This probably indicates the spatial expansion of saline subtropical gyre waters due to enhanced Ekman downwelling and might also point to a changed density structure and altered geostrophic balance in Florida Straits. Notably, these oceanographic alterations are not consistently occurring during periods of AMOC reduction. The data suggest that the expansion of gyre waters into Florida Straits was impeded by the increasing influence of Antarctic Intermediate Water (AAIW) from MIS 5.5 to ∼107 kyr BP. Afterward, increasingly positive benthic d13C values imply a recession of AAIW, allowing the temporary expansion of gyre waters into Florida Straits. We argue that the inferred transient subtropical salt accumulation and warm pool expansion might have played a pivotal role in reinvigorating meridional overturning and dampen the severity of interglacial cold phases.