Bottom Water Ventilation Research Articles

Climate cyclicity-controlled recurrent bottom-water ventilation events in the aftermath of the Toarcian Oceanic Anoxic Event: the Jenkyns Event

AbstractEnvironmental perturbations of the Toarcian Anoxic Event and its associated carbon isotope excursion (CIE) occurred in a cyclic fashion indicating an orbital control mechanism. Sedimentary strata of the E. elegantulum ammonite subzone in the Lorraine Sub-basin, Luxembourg, exhibit eight sedimentary cycles, most of which postdate the CIE, implying that its termination did not coincide with a full recovery from environmental stress. Sea-level and temperature fluctuations of the Toarcian crisis were linked to a cryosphere demise in the Northern Hemisphere, which modulated stadial versus interstadial phases on the orbital 100 ka eccentricity frequency band. Upon stadial phases, enhanced wind strength in combination with lowered sea level disrupted stratified shelf waters and shifted the storm wave base close to the sea floor. Ventilation of bottom waters interrupted accumulation of laminated and organic-rich black shales, but formed organic-lean and non-laminated gray claystones enriched in terrigenous wax lipids and spores provided via aeolian transport from the hinterland due to the enhanced wind strength. Wind systems are assumed to have been driven by high pressure differences between the cryosphere in the North of the Tethyan shelf and the warmer costal lowland. This distinguishes the atmospheric turbulences after the CIE from the postulated intensification of the cyclones that accompanied the rapid warming at the beginning of the CIE. The deep-water ventilation event following the CIE in the Lorraine Sub-basin was accompanied by a parallel evolution in the SW German Basin, indicating a supraregional driving mechanism in paleobathymetric sub-basins susceptible to lowering of storm wave base. In sub-basins with greater water depth on the NW-Tethyan shelf, post-CIE orbitally driven atmospheric turbulences had a minor or no effect on water column stratification and deposition of organic-rich laminated black shales persisted. Graphical abstract

Benthic Foraminiferal Response to the Aptian−Albian Carbon Cycle Perturbation in the Atlantic Ocean

Abstract A planktic foraminiferal mass extinction, coeval with the major carbon cycle perturbation of Oceanic Anoxic Event (OAE) 1b, occurred at the Aptian−Albian boundary interval (AABI). However, the scarcity of high-resolution records across the AABI hampers an assessment of the impacts of OAE 1b on deep-water benthic foraminiferal assemblages. Here we present high-resolution benthic foraminiferal census counts at Deep Sea Drilling Project (DSDP) Site 511 (southern South Atlantic Ocean) and Ocean Drilling Program (ODP) Site 1049 (western subtropical North Atlantic Ocean) over the AABI. Our records at these bathyal sites provide conclusive evidence that there was no benthic foraminiferal extinction at the Aptian−Albian boundary, although marked reorganizations of relative abundances occurred. During the latest Aptian, cyclic increases in the abundance of infaunal species at both sites point to repeated pulses of reduced bottom water oxygenation and increased organic carbon flux to the ocean floor. Additionally, agglutinated and weakly calcified benthic foraminiferal species were relatively abundant during the latest Aptian, suggesting deep-water carbonate ion depletion in the Atlantic Ocean, although we did not identify signs of carbonate dissolution at these relatively shallow sites. At Site 511, abundances of infaunal foraminifera increased in tandem with the negative carbonate carbon isotope (δ13Ccarb) excursion of the Kilian sub-event within OAE 1b, suggesting decreased bottom water ventilation and increased organic carbon flux to the ocean floor during the sub-event. Bottom water ventilation and carbonate ion saturation improved during the earliest Albian in the Atlantic Ocean, followed by high-amplitude oscillations, as suggested by abundance trends of heavily calcified epifaunal foraminifera at Sites 511 and 1049.

Stagnant North Atlantic Deep Water Heat Uptake With Reduced Atlantic Meridional Overturning Circulation During the Last Deglaciation

AbstractAtlantic Meridional Overturning Circulation (AMOC) plays a major role in the climate system by modulating the depth and rate of oceanic heat storage. Some climate simulations suggest that reduced AMOC decreases bottom water ventilation and that the heat absorbed by the ocean starts to mix downwards, warming Atlantic intermediate waters. This has been corroborated for the western North Atlantic by benthic foraminifera geochemical records from periods of reduced AMOC during the last deglaciation. However, the deep‐water response remains poorly constrained, and the lack of direct paleotemperature reconstructions limits our understanding about the effects of reduced circulation on ocean heat uptake. We present a new reconstruction of bottom water temperatures from core GeoB9508‐5 (2,384 m water depth, 15°29.90°N/17°56.88°W) off the northwestern African Margin. Our paleotemperature record, based on Uvigerina spp. Mg/Ca, shows two episodes of intense transient deep water warming in times of decreasing overturning circulation, followed by long periods of heat uptake stagnation. First, during AMOC slowdown in the Heinrich stadial 1, when paleotemperatures of ∼2°C persisted for ∼5.4 Kyr coincident with the weakest stage of AMOC; and second in the Younger Dryas, when bottom water temperatures >4°C lasted ∼2.5 Kyr during a less intense AMOC decline. This suggests a stagnation of deep‐water heat uptake in the deep NE Atlantic possibly linked to a reduced downward advection of heat during times of a reduced AMOC, supporting the hypothesis that AMOC strength sets the depth of oceanic heat storage in the North Atlantic.

Parametric Subharmonic Instability of Diurnal Internal Tides in the Abyssal South China Sea

Abstract Internal waves close to the seafloor of abyssal oceans are the key energy suppliers driving near-bottom mixing and the upwelling branches of meridional overturning circulation, but their spatiotemporal variability and intrinsic mechanisms remain largely unclear. In this study, measurements from 10 long-term moorings were used to investigate the internal wave activities in the abyssal South China Sea, which is an important upwelling zone. Strong near-inertial internal waves (NIWs) with current velocity pulses exceeding 5 cm s−1 were observed to dominate the near-bottom internal wave field at approximately 14°N. These abyssal NIWs were phase-coupled with diurnal internal tides (D1), and both displayed common seasonal variations that were larger in winter and summer, providing evidence of diurnal parametric subharmonic instability (PSI) near its critical latitudes (CLs). Emitted from the bottom, near-inertial kinetic energy rapidly decreased by one order of magnitude from depths of ∼120 to ∼620 m above the bottom. Near rough topographies, the abyssal PSI was shifted poleward to approximately 14.8°N by negative relative vorticities of passing anticyclonic eddies or topographic Rossby waves. Compared with flat topography, PSI near rough topography was significantly promoted by topographic-localized strong D1 with high-mode structures, creating abyssal NIW bursts. Bottom-reaching shipboard conductivity–temperature–depth profiles revealed that the bottom mixed layers became much thicker when approaching CLs, suggesting that abyssal PSI potentially accelerates the ventilation and upwelling of bottom water. The observational results presented here illustrate notable spatiotemporal variations in abyssal NIWs regulated by PSI and call for consideration of PSI to better understand near-bottom mixing and upwelling.

Controls on organic matter accumulation on the Late-Ordovician shales in Awati Sag in northwestern Tarim Basin, NW China

The black shales of the Upper Ordovician Yingan Formation (Fm.) in the northwestern margin of the Tarim Basin was previously considered to be an important source rock. However, the total organic carbon (TOC) values of Yingan black shales was low, averaging 0.51%. Therefore, it remains debated whether they can be served as high-quality source rock. Here, we investigated the sedimentary environment (including terrigenous input, chemical weathering intensity, paleoproductivity, paleoredox conditions) of the Yingan shales and the factors affecting organic matter (OM) enrichment through organic and inorganic geochemical methods. The proxies of terrigenous flux (e.g., Ti/Al and Zr/Al) and sedimentation rate flux [e.g., (La/Yb)P] shows that the terrigenous inputs has no effect on OM enrichment. The low primary productivity (e.g., low values of P/Al, Ba/Al and Ni/Al) of the Yingan shales also exhibits a little effect on OM enrichment. However, the paleoredox proxies (e.g., U/Al, V/Al, U/Th and V/Cr) show that the black shale was deposited in a partial oxidation environment of bottom waters, which is unfavorable for the preservation and enrichment of OM evidenced. The oxidation environment was caused by the enhanced thermohaline circulation and ventilation of bottom waters due to the increasing cooling global climate during Late Ordovician. Therefore, the Yingan shales cannot be served as a source rock or shale gas reservoir. Additionally, the poor OM enrichment model provides new insights for the evaluation of shales linked to cold climatic conditions.

Episodic ventilation of euxinic bottom waters triggers the formation of black shale-hosted Mn carbonate deposits

Many manganese (Mn) carbonate deposits over geologic history are often closely associated with black shales. However, the mechanism(s) by which the Mn entered the sediments remains controversial because on one hand oxygenated waters are required for primary Mn(IV) oxide formation but on the other hand black shales generally form under reducing conditions. China hosts some of the world’s largest black shale-hosted Mn deposits, for example, the Carboniferous Ortokarnash and Malkantu Mn deposits in the Malkansu region, offering a good opportunity to revisit the genesis of black shale-hosted Mn carbonate deposits. Here, we present the first coupled, systematic paleoredox and hydrographic reconstruction of those deposits with the aim of elucidating the redox-state of the depositional basin at the time of their initial deposition. The Mn ore beds in this region are hosted within laminated, organic-rich mudstones (i.e., black shales) reflective of a relatively deep-water depositional environment. The Mn ores are comprised of the Mn(II) carbonate minerals (rhodochrosite and Ca-rhodochrosite), with minor alabandite, pyrite, and monazite. Multiple independent lines of evidence, including positive shale-normalized Ce anomalies (average 3.0), negative δ13CVPDB values (average −11.55 ‰), and negative δ98MoNIST+0.25 values (average −1.07 ‰), indicate that the Mn(II) carbonate ores were formed during diagenesis via the coupled oxidation of organic matter and reduction of Mn(IV) oxides originally deposited from an oxygenated water column. However, in apparent contradiction, highly reactive iron to total iron (FeHR/FeT; average 0.66) and pyrite iron to highly reactive iron (FePy/FeHR; average 0.71) ratios, combined with a high abundance of small framboidal pyrites (mean diameter ∼5 μm) with narrow size ranges (standard deviations <2 μm), suggest that the associated black shales were deposited in euxinic (H2S-bearing) bottom waters. Euxinia resulted from sluggish water mass circulation, with sedimentary Mo/TOC ratios indicating a relatively strong hydrologic restriction. During this stage, dissolved Mn(II) would have accumulated in euxinic waters but not become permanently fixed into sediments, as indicated by higher degrees of enrichment for Mo over U and lower Mo isotope values than coeval seawater recorded in the black shales. These reveal an active Mn(IV) oxide shuttle across a redox-stratified water column. Combined, the Mn ore intervals document sharp benthic oxygenation of euxinic bottom waters, a process that might be induced by the periodic incursions of oxic seawater associated with eustatic sea level rises. This model is supported by the Mo isotopic characteristics of the Mn carbonate ores, which indicate original Mn oxide precipitation from an overlying water with Mo isotopic composition close to coeval open seawater (minimum δ98MoNIST+0.25 of 1.9 ‰). In contrast to the generalized “bathtub ring” model envisaging that the black shale-hosted Mn deposits were formed at shallow basin margins, our results highlight that they were more likely formed by in-situ ventilation of anoxic (especially euxinic) deep waters of basin center settings. In this regard, we suggest that the ventilation model better explains the close association of Mn carbonate ores with black shales, and has profound implications for understanding the paleoredox framework of otherwise Mn-rich black shale successions.

Seasonal controls on nearshore dissolved oxygen variability and hypoxia in a coastal embayment

Declining dissolved oxygen (DO) is emerging as an increasingly important stressor in nearshore ecosystems, and there is a growing need to better understand DO dynamics and hypoxia risk in this highly variable environment. In this study, we collected data from monthly cruises on the inner shelf, continuous nearshore moorings inside and outside a small coastal upwelling embayment (San Luis Obispo Bay in Central California), and weekly phytoplankton measurements inside the bay during the upwelling season. Nearshore DO was generally dominated by low-frequency synoptic variability, with increased DO variance near the surface relative to the bottom and inside the bay compared to outside. Two nearshore hypoxic regimes were identified. In the first regime, which occurred during periods of strong upwelling in the spring across all nearshore sites, the nearshore bottom water temperature-DO (T-DO) relationship was aligned with that found offshore, suggesting hypoxia was driven by the direct advection and cross-shelf exchange of low DO subthermocline waters from the shelf. This period also coincided with minimal water-column stratification, small vertical DO differences, and a diatom-dominated phytoplankton assemblage. In the second regime, which occurred during summer months and was characterized by weaker upwelling, strong stratification, and dinoflagellate-dominated phytoplankton assemblage, the near-bottom T-DO relationship inside the bay deviated significantly from that on the shelf offshore. These hypoxic events inside the bay were likely driven by localized respiration and lack of ventilation of bottom waters due to strong stratification. Collectively, these observations reveal a shift in the strength and magnitude of physical versus biological processes driving nearshore DO dynamics. The high spatiotemporal variability of DO dynamics in upwelling bays means that they are likely to be at the forefront of ecosystem impacts of and adaptions to climate change, and may act as sentinel systems or “canaries on the coast.”

Rapid vertical exchange at fronts in the Northern Gulf of Mexico.

Over the Texas-Louisiana Shelf in the Northern Gulf of Mexico, the eutrophic, fresh Mississippi/Atchafalaya river plume isolates saltier waters below, supporting the formation of bottom hypoxia in summer. The plume also generates strong density fronts, features of the circulation that are known pathways for the exchange of water between the ocean surface and the deep. Using high-resolution ocean observations and numerical simulations, we demonstrate how the summer land-sea breeze generates rapid vertical exchange at the plume fronts. We show that the interaction between the land-sea breeze and the fronts leads to convergence/divergence in the surface mixed layer, which further facilitates a slantwise circulation that subducts surface water along isopycnals into the interior and upwells bottom waters to the surface. This process causes significant vertical displacements of water parcels and creates a ventilation pathway for the bottom water in the northern Gulf. The ventilation of bottom water can bypass the stratification barrier associated with the Mississippi/Atchafalaya river plume and might impact the dynamics of the region’s dead zone.

Reflooding and repopulation of the Mediterranean Sea after the Messinian Salinity Crisis: Benthic foraminifera assemblages and stable isotopes of Spanish basins

Benthic foraminiferal, sedimentological, and stable isotope analyses performed on early Zanclean sediments from Alboran Basin ODP Site 976 and southern Spanish land-based sections in the Malaga, Nijar and Sorbas basins have enabled the reconstruction of Mediterranean environmental conditions immediately after the Messinian Salinity Crisis. The presence at the Miocene – Pliocene boundary of dark layers, often enriched in organic matter, suggests that the Zanclean reflooding has created water column stratification, and reduced bottom-water oxygen levels. Considering that such dark layers are recorded at both deep and marginal settings far away from the Gibraltar gateway/s, a Mediterranean-scale water-mass stratification must have occurred. This stratification could be the result of saline Atlantic waters sinking into a less saline Mediterranean Basin still under the influence of the Paratethys. Our early Zanclean benthic δ18O data show that the Mediterranean water budget was indeed less negative than at present, explaining the lower salinity of the basin. However, the Atlantic values of the benthic δ13C registered in the Alboran basin suggest that bottom-water renewal rates were quite high during the early Zanclean, preventing the reduction of δ13C at the seafloor as observed in the Messinian records. Zanclean benthic foraminiferal repopulation sequences show similarities with recovery from low-oxic episodes during sapropel deposition. These observations, paired with the gradual deepening of the basins, suggests that the Zanclean reflooding led to a progressive shift from stressed and unstable environments towards benthic associations typical of efficient circulation and bottom water ventilation.

Seasonal hypoxia and temperature inversions in a tropical bay

AbstractDissolved oxygen (DO) is a critically important ecological variable, and the prevalence of marine hypoxia is expected to increase due to the combined effects of ocean warming and eutrophication. Thermal stress can co‐occur with hypoxia, especially in tropical systems, and can exacerbate its effects. We examine the physical processes that are important in regulating hypoxia and temperature inversions in Bahía Almirante, a semi‐enclosed tropical embayment on the Caribbean coast of Panama. A 10‐yr record of observations at 7 locations within Bahía Almirante reveals seasonal temperature inversions and hypoxia at depth that often co‐occur. These features are more severe and persistent in the back bay, though they occur throughout Bahía Almirante. DO reductions correspond to periods with high freshwater input, including direct precipitation, resulting in strong salinity stratification that isolates bottom waters, allowing biological oxygen demand to draw down DO. Evidence indicates that lateral advection can contribute to reoxygenation events, and the relationship between near bottom DO and bottom salinities in the mid bay and back bay is consistent with deep‐water renewal as the mechanism for bottom water ventilation. These hypoxia and temperature inversion events impact the biological communities of Bahía Almirante, and the physical dynamics that regulate these coincident and persistent stressors for marine organisms are likely present in other shallow, tropical estuaries.

Simulation of Winter Deep Slope Convection in Peter the Great Bay (Japan Sea)

In wintertime, a high-density water forms on the shallow shelf in the vast Peter the Great Bay (Japan Sea). The steep continental slope with deep canyons and cold winters in the area provide suitable conditions for the implementation of deep slope convection—an important phenomenon in the formation of intermediate and bottom waters that occurs at a few locations in some semi-enclosed seas, including the Japan Sea. The descent of dense shelf water down the continental slope of Peter the Great Bay usually occurs to 1000–1200 m; however, in anomalously cold winters, it has been observed at greater than 2000 m depth supporting renewal and deep ventilation of intermediate and bottom waters in the Japan Sea. The deep slope convection is a rare episodic phenomenon with durations ranging from several hours to several days, that has never been simulated in Peter the Great Bay with a realistic numerical model of circulation. We apply the Regional Ocean Modeling System (ROMS) with a 600 m horizontal resolution to simulate the deep slope convection in the anomalously cold winter of 2001 when it has been observed in cruises. The results are compared with propagation of deep shelf water in the regular winter of 2010 when hydrological characteristics of this water were recorded by a profiler “Aqualog” installed at the shelf break. Using Lagrangian methods, we track and analyze the formation of dense shelf water, its advection to the slope edge in the bottom layer and descent down the slope. Special attention is payed to the role of coastal eddies arising due to a symmetric instability. These eddies promote the cross-shelf transport of the dense shelf water towards the continental slope edge. The simulation results are compared with rare observations of the deep slope convection in Peter the Great Bay.

The response of calcareous plankton to the Sapropel S1 interval in North Ionian Sea

A high resolution study of calcareous nannofossils has been performed on samples from the Sapropel S1interval deposited in the North Ionian Sea, with the aim to assess the paleoenvironmental changes in the photic zone during this crucial interval in Mediterranean circulation. Calcareous nannofossil data have been integrated with planktonic foraminiferal data recently published from which the paleoclimatic curve has been constructed. Placoliths (namely Emiliania huxleyi) and Florisphaera profunda distributions, along with that of planktonic foraminifer Globigerinoides ruber white, evidence that, after a progressive weakening of surface water mixing, a deep chlorophyll maximum developed just prior to the sapropel deposition. We suggest that these changes took place as a response to enhanced precipitation conditions and riverine discharge as testified by increasing trend of reworked coccoliths and the occurrence of lower salinity taxon Braarudosphaera bigelowii. Calcareous nannofossils also point out that the oceanographic (water column stratification, reduced bottom water ventilation) and biogeochemical (increased primary production) processes that occurred during the S1 formation were particularly dominant during the earliest part of the older S1 warm phase (S1a). Our results support than some re-ventilation events of the shallow depth of studied site (665 m) occurred to some extent, particularly during the final phases of S1a. The distribution of holococcoliths, more abundant during the cold interruption phase S1i, seems confirm that the preservation of these tiny and delicate coccoliths, highly susceptible to dissolution, is enhanced under seafloor re-ventilation conditions. Finally, we tentatively suggest that preservation also plays a significant role in the distribution of the warm upper photic zone taxa, particularly during the warm S1b interval.

Retracing hypoxia in Eckernförde Bight (Baltic Sea)

Abstract. An increasing number of dead zoning (hypoxia) has been reported as a consequence of declining levels of dissolved oxygen in coastal oceans all over the globe. Despite substantial efforts a quantitative description of hypoxia up to a level enabling reliable predictions has not been achieved yet for most regions of societal interest. This does also apply to Eckernförde Bight (EB) situated in the Baltic Sea, Germany. The aim of this study is to dissect underlying mechanisms of hypoxia in EB, to identify key sources of uncertainties, and to explore the potential of existing monitoring programs to predict hypoxia by developing and documenting a workflow that may be applicable to other regions facing similar challenges. Our main tool is an ultra-high spatially resolved general ocean circulation model based on a code framework of proven versatility in that it has been applied to various regional and even global simulations in the past. Our model configuration features a spacial horizontal resolution of 100 m (unprecedented in the underlying framework which is used in both global and regional applications) and includes an elementary representation of the biogeochemical dynamics of dissolved oxygen. In addition, we integrate artificial “clocks” that measure the residence time of the water in EB along with timescales of (surface) ventilation. Our approach relies on an ensemble of hindcast model simulations, covering the period from 2000 to 2018, designed to cover a range of poorly known model parameters for vertical background mixing (diffusivity) and local oxygen consumption within EB. Feed-forward artificial neural networks are used to identify predictors of hypoxia deep in EB based on data at a monitoring site at the entrance of EB. Our results consistently show that the dynamics of low (hypoxic) oxygen concentrations in bottom waters deep inside EB is, to first order, determined by the following antagonistic processes: (1) the inflow of low-oxygenated water from the Kiel Bight (KB) – especially from July to October – and (2) the local ventilation of bottom waters by local (within EB) subduction and vertical mixing. Biogeochemical processes that consume oxygen locally are apparently of minor importance for the development of hypoxic events. Reverse reasoning suggests that subduction and mixing processes in EB contribute, under certain environmental conditions, to the ventilation of the KB by exporting recently ventilated waters enriched in oxygen. A detailed analysis of the 2017 fish-kill incident highlights the interplay between westerly winds importing hypoxia from KB and ventilating easterly winds which subduct oxygenated water.

Deglacial to Holocene environmental changes in the northern Ligurian Sea: The dual influence of regional climate variability and large-scale intermediate Mediterranean circulation

The sedimentary archives of the Mediterranean Sea record periodic deposits of organic-rich deposits, called sapropels in the eastern basin and organic-rich layers (ORL) in the western basin. Changes in both the Mediterranean circulation and inputs of fresh water through borderlands rivers under more humid climate, are important mechanisms to explain those events. The last ORL and sapropel S1 have different timing, respectively from ~14.5 to 9 Ka and from ~10 to 6 Ka, presumably due to different forcing factors in the western basin (i.e., melting of Alps ice). Here we present a high-resolution study of a marine sediment core located off the mouth of the Var River, one of the most dynamic river system of the northern borderland of the western Mediterranean Sea. We applied a multi-proxy approach based on benthic foraminiferal assemblages, foraminiferal δ18O and δ13C, grain size analyses, organic carbon content and XRF elemental data to decipher the regional climate signals from the basin-scale intermediate circulation signature. Our results do not show large river inputs at the timing of the last ORL deposit. On the opposite, foraminiferal and geochemical evidence indicate that the 11–6 kyr period, concomitant to Sapropel S1 event in the Eastern Mediterranean, was characterised by high river activity and low ventilated bottom waters at the studied location. Additionally, our results characterized the last 6 ka with large scale episodes of more active bottom water ventilation due perhaps to enhanced wind activity under an overall cooler climate.Time series analyses were computed from stable isotopes, Ca/Ti XRF ratio and foraminifera abundances. They show common frequencies peaks (2.2–3, 1.1–1.2, 0.9–1.0, and 0.4–0.5 kyr) most likely related to the solar activity. Also, a specific frequency band (1.5–1.6 kyr) was only recorded in benthic foraminiferal abundance and stable isotopic records. This was preferentially attributed to an oceanic-driven internal forcing.

Ventilation History of the Nordic Seas Deduced From Pelagic‐Benthic Radiocarbon Age Offsets

AbstractChanges in ocean circulation are considered a major driver of centennial‐to‐millennial scale climate variability during the last deglaciation. Using four sediment records from the Nordic Seas, we studied radiocarbon ventilation ages in subsurface and bottom waters to reconstruct past variations in watermass overturning. Planktic foraminiferal ages show significant spatial variability over most of the studied period. These differences suggest that the ventilation of the shallower subsurface waters is strongly influenced by local conditions such as sea‐ice and meltwater input, changes in mixed‐layer depth, and/or variable contributions of water masses with different 14C signatures. Despite covering a significant water depth range, the benthic foraminiferal records show common long‐term patterns, with generally weaker ventilation during stadials and stronger during interstadials. The Greenland Sea record differs the most from the other records, which can be explained by the greater depth and the geographical distance of this site. The benthic records reflect regional shifts in deep convection and suggest that the deep Nordic Seas have been generally bathed by a single, though changing, deep‐water mass analogous to the present‐day Greenland Sea Deep Water. Since significant offsets in ventilation ages are yielded by different taxonomic or ecological groups of benthic foraminifera, the use of uniform material seems a prerequisite to reconstruct bottom water ventilation histories.

Ventilation of the abyss in the Atlantic sector of the Southern Ocean

The Atlantic sector of the Southern Ocean is the world’s main production site of Antarctic Bottom Water, a water-mass that is ventilated at the ocean surface before sinking and entraining older water-masses—ultimately replenishing the abyssal global ocean. In recent decades, numerous attempts at estimating the rates of ventilation and overturning of Antarctic Bottom Water in this region have led to a strikingly broad range of results, with water transport-based calculations (8.4–9.7 Sv) yielding larger rates than tracer-based estimates (3.7–4.9 Sv). Here, we reconcile these conflicting views by integrating transport- and tracer-based estimates within a common analytical framework, in which bottom water formation processes are explicitly quantified. We show that the layer of Antarctic Bottom Water denser than 28.36 kg m^{-3}gamma _{n} is exported northward at a rate of 8.4 ± 0.7 Sv, composed of 4.5 ± 0.3 Sv of well-ventilated Dense Shelf Water, and 3.9 ± 0.5 Sv of old Circumpolar Deep Water entrained into cascading plumes. The majority, but not all, of the Dense Shelf Water (3.4 ± 0.6 Sv) is generated on the continental shelves of the Weddell Sea. Only 55% of AABW exported from the region is well ventilated and thus draws down heat and carbon into the deep ocean. Our findings unify traditionally contrasting views of Antarctic Bottom Water production in the Atlantic sector, and define a baseline, process-discerning target for its realistic representation in climate models.

Nannofossil palaeoecology of Lower Miocene sapropels from IODP Expedition 359, the Maldives

During IODP expedition 359 multiple holes recovered Modern to Middle Miocene sediments from the Kardiva Drift which is strongly influenced by monsoonally-driven currents. Two holes continued below the drift into Lower Miocene sediments that were deposited under rather different conditions following Oligocene subsidence of the Maldives Ridge. These sediments are characterised by marked alternation between dark organic-rich sapropelic sediments and pale nannofossil oozes. This interval is also characterised by generally good to excellent nannofossil preservation. Nannofossil assemblages from the sapropel and non-sapropel intervals are broadly similar; both sediment types contain diverse normal marine assemblages and no taxa are confined to either sediment type. In the sapropels nannofossil abundance and diversity tends to be lower and Cyclicargolithus floridanus specimens are smaller. In addition Umbilicosphaera abundances are higher in the non-sapropel layers whilst Discoaster abundance is higher in most sapropels. These differences are more marked in the better developed sapropels. The nannofossil evidence supports the interpretation from geochemistry that sapropel formation was primarily driven by changes in bottom water ventilation linked to sea-level change rather than surface water productivity.

On the Potential Optical Signature of Convective Turbulence over the West Florida Shelf

Atmospheric cold front propagation across the northern Gulf of Mexico is characterized by elevated surface wind velocities and a ~10–15 °C drop in surface air temperatures. These meteorological conditions result in significant heat energy losses from the surface ocean to the overlying atmosphere. These seasonally recurring cold-air outbreak events may penetrate the southern portion of the West Florida continental shelf and initiate turbulent and convective overturn of the water column. Examination of true color images derived from ocean-viewing, satellite-based radiometer data reveals coincident and substantial surface water discolorations that are optically similar to smaller-scale “whiting events,” despite the regional-scale extent of the observed phenomenon (&gt;25,000 km2). Coupled air–sea numerical simulations suggest the surface water discoloration occurs and is sustained where the entire water column is dynamically unstable. The simulation results indicate significant density (σt) inversions between the surface and bottom waters. Thus, the combined numerical model and remote sensing analysis suggest that convective turbulence may be contributing to the sustained ventilation of bottom waters containing a high concentration of suspended particulates. High-temporal resolution true color images rendered from the GOES-R Advanced Baseline Imager (ABI) data appear to support the surface water discoloration’s turbulent-driven nature.

Mediterranean Outflow Water dynamics across the middle Pleistocene transition based on a 1.3 million-year benthic foraminiferal record off the Portuguese margin

Reconstruction of the Mediterranean Outflow Water (MOW) dynamics in the geologic past contributes to exploring its influence on the North Atlantic Ocean circulation and global climate. The middle Pleistocene transition (MPT) is one of the most important characteristics of Quaternary climate change, but until now, research on the MOW variability during this time interval has been limited. To decipher the MOW variability over the last 1.3 myr, this work presents the first continuous and high-resolution (∼2 kyr) foraminiferal stable oxygen and carbon isotopes and benthic foraminiferal records from Integrated Ocean Drilling Program (IODP) Site U1391, which was drilled off the Portuguese margin in the Northeast Atlantic Ocean and is currently bathed by the lower MOW core. Based on spectral analysis results of the foraminiferal δ18O record, the MPT peaked from 0.9 to 0.7 Ma. The ‘epibenthos group’ was previously found attached to elevated substrates only within the MOW current and more abundant at higher current velocities, so the relative abundance of the ‘elevated epibenthos group’ was adopted as an indicator of MOW strength in this study. The MOW indicator suggests that the MOW strengthened during MIS 1, MIS 5, MIS 7, MIS 9, MIS 11, MIS 13, MIS 15, MIS 17, early MIS 18 to late MIS 19, MIS 21, MISs 23–25, MISs 28–30, early MIS 34 to MIS 35, early MIS 36 to late MIS 37 and MIS 40 with a ∼100 kyr cycle. Superimposed on the long-term variability of the MOW current, the ∼41-kyr and ∼20-kyr cyclic fluctuations are also evident, whereby the ∼41-kyr cycle is more significant. The results of cross-spectral analyses indicate that the ∼20-kyr cyclic variation in the MOW intensity was drived by the precession forcing; the ∼41-kyr cyclic variation in the pre- MPT MOW dynamics and the ∼100-kyr cyclic variation in the post-MPT MOW dynamics were strongly controlled by the Earth’s obliquity. The variation in the abundance of the elevated epibenthos group was dominated by the ∼41-kyr cycle prior to the MPT and by the ∼100-kyr cycle after the MPT, which suggested there was a MPT-related shift in the MOW dynamics. Benthic δ13C record of the last 0.56 myr shows precessional (23-kyr) and glacial-interglacial (∼100 kyr) cycles, indicating cyclic variations in bottom water ventilation. Enhanced ventilation coincides with precession maxima and reduced ventilation with precession minima. Bottom water was well ventilated by MOW during the interglacial periods and by GNAIW during the glacial periods, and pooly ventilated during the glaical terminations.

Ventilation of the northern Baltic Sea

Abstract. The Baltic Sea is a semi-enclosed, brackish water sea in northern Europe. The deep basins of the central Baltic Sea regularly show hypoxic conditions. In contrast, the northern parts of the Baltic Sea, the Bothnian Sea and Bothnian Bay, are well oxygenated. Lateral inflows or a ventilation due to convection are possible mechanisms for high oxygen concentrations in the deep water of the northern Baltic Sea. In March 2017, conductivity–temperature–depth (CTD) profiles and bottle samples, ice core samples, and brine were collected in the Bothnian Bay. In addition to hydrographic standard parameters, light absorption has been measured in all samples. A complementary numerical model simulation provides quantitative estimates of the spread of newly formed bottom water. The model uses passive and age tracers to identify and trace different water masses. Observations indicate a recent ventilation of the deep bottom water at one of the observed stations. The analysis of observations and model simulations shows that the Bothnian Bay is ventilated by dense water formed due to mixing of Bothnian Sea and Bothnian Bay surface water initializing lateral inflows. The observations show the beginning of the inflow and the model simulation demonstrates the further northward spreading of bottom water. These events occur during wintertime when the water temperature is low. Brine rejected during ice formation barely contributes to dense bottom water.