Age model and Nd isotope ratios of DSDO Hole 43-386 and ODP Hole 210-1276A

Modern thermohaline circulation plays a role in latitudinal heat transport and in deep-ocean ventilation, yet ocean circulation may have functioned differently during past periods of extreme warmth, such as the Cretaceous. The Late Cretaceous (100-65 Ma) was an important period in the evolution of the North Atlantic Ocean, characterized by opening ocean gateways, long-term climatic cooling and the cessation of intermittent periods of anoxia (oceanic anoxic events, OAEs). However, how these phenomena relate to deep-water circulation is unclear. We use a proxy for deep-water mass composition (neodymium isotopes; e-Nd) to show that, at North Atlantic ODP Site 1276, deep waters shifted in the early Campanian (~78-83 Ma) from e-Nd values of ~-7 to values of ~-9, consistent with a change in the style of deep-ocean circulation but >10 Myr after a change in bottom water oxygenation conditions. A similar, but more poorly dated, trend exists in e-Nd data from DSDP Site 386. The Campanian e-Nd transition observed in the North Atlantic records is also seen in the South Atlantic and proto-Indian Ocean, implying a widespread and synchronous change in deep-ocean circulation. Although a unique explanation does not exist for the change at present, we favor an interpretation that invokes Late Cretaceous climatic cooling as a driver for the formation of Southern Component Water, which flowed northward from the Southern Ocean and into the North Atlantic and proto-Indian Oceans.

Haemolymph inorganic ion composition, physiology and behaviour dependent on temperature and magnesium in adult Paralomis granulosa (Decapoda, Anomura, Lithodidae)

A low capacity for regulation of extracellular Mg2+ has been proposed to exclude reptant marine decapod crustaceans from temperatures below 0°C and thus to exclude them from the high Antarctic. To test this hypothesis and to elaborate the underlying mechanisms in the most cold-tolerant reptant decapod family of the sub-Antarctic, the Lithodidae, thermal tolerance was determined in the crab Paralomis granulosa (Decapoda, Anomura, Lithodidae) using an acute stepwise temperature protocol (-1°, 1°, 4°, 7°, 10°, and 13°C). Arterial and venous oxygen partial pressures (Po2) in hemolymph, heartbeat and ventilation beat frequencies, and hemolymph cation composition were measured at rest and after a forced activity (righting) trial. Scopes for heartbeat and ventilation beat frequencies and intermittent heartbeat and scaphognathite beat rates at rest were evaluated. Hemolymph [Mg2+] was experimentally reduced from 30 mmol/L to a level naturally observed in Antarctic caridean shrimps (12 mmol/L) to investigate whether the animals remain more active and tolerant to cold (-1°, 1°, and 4°C). In natural seawater, righting speed was significantly slower at -1° and 13°C, compared with acclimation temperature (4°C). Arterial and venous hemolymph Po2 increased in response to cooling even though heartbeat and ventilation beat frequencies as well as scopes decreased. At rest, ionic composition of the hemolymph was not affected by temperature. Activity induced a significant increase in hemolymph [K+] at -1° and 1°C. Reduction of hemolymph [Mg2+] did not result in an increase in activity, an increase in heartbeat and ventilation beat frequencies, or a shift in thermal tolerance to lower temperatures. In conclusion, oxygen delivery in this cold-water crustacean was not acutely limiting cold tolerance, and animals may have been constrained more by their functional capacity and motility. In contrast to earlier findings in temperate and subpolar brachyuran crabs, these constraints remained insensitive to changing Mg2+ levels.

Records of past mid-depth ventilation: Cretaceous ocean anoxic event 2 vs. recent oxygen minimum zones

Present day oceans are well ventilated, with the exception of mid-depth oxygen minimum zones (OMZs) under high surface water productivity, regions of sluggish circulation, and restricted marginal basins. In the Mesozoic, however, entire oceanic basins transiently became dysoxic or anoxic. The Cretaceous ocean anoxic events (OAEs) were characterised by laminated organic-carbon rich shales and low-oxygen indicating trace fossils preserved in the sedimentary record. Yet assessments of the intensity and extent of Cretaceous near-bottom water oxygenation have been hampered by deep or long-term diagenesis and the evolution of marine biota serving as oxygen indicators in today's ocean. Sedimentary features similar to those found in Cretaceous strata were observed in deposits underlying Recent OMZs, where bottom-water oxygen levels, the flux of organic matter, and benthic life have been studied thoroughly. Their implications for constraining past bottom-water oxygenation are addressed in this review. We compared OMZ sediments from the Peruvian upwelling with deposits of the late Cenomanian OAE 2 from the north-west African shelf. Holocene laminated sediments are encountered at bottom-water oxygen levels of < 7 µmol/kg under the Peruvian upwelling and < 5 µmol/kg in California Borderland basins and the Pakistan Margin. Seasonal to decadal changes of sediment input are necessary to create laminae of different composition. However, bottom currents may shape similar textures that are difficult to discern from primary seasonal laminae. The millimetre-sized trace fossil Chondrites was commonly found in Cretaceous strata and Recent oxygen-depleted environments where its diameter increased with oxygen levels from 5 to 45 µmol/kg. Chondrites has not been reported in Peruvian sediments but centimetre-sized crab burrows appeared around 10 µmol/kg, which may indicate a minimum oxygen value for bioturbated Cretaceous strata. Organic carbon accumulation rates ranged from 0.7 and 2.8 g C /cm2 /kyr in laminated OAE 2 sections in Tarfaya Basin, Morocco, matching late Holocene accumulation rates of laminated Peruvian sediments under Recent oxygen levels below 5 µmol/kg. Sediments deposited at > 10 µmol/kg showed an inverse exponential relationship of bottom-water oxygen levels and organic carbon accumulation depicting enhanced bioirrigation and decomposition of organic matter with increased oxygen supply. In the absence of seasonal laminations and under conditions of low burial diagenesis, this relationship may facilitate quantitative estimates of palaeo-oxygenation. Similarities and differences between Cretaceous OAEs and late Quaternary OMZs have to be further explored to improve our understanding of sedimentary systems under hypoxic conditions.

On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic

Ocean observations carried out in the framework of the Collaborative Research Center 754 (SFB 754) "Climate-Biogeochemistry Interactions in the Tropical Ocean" are used to study (1) the structure of tropical oxygen minimum zones (OMZs), (2) the processes that contribute to the oxygen budget, and (3) long-term changes in the oxygen distribution. The OMZ of the eastern tropical North Atlantic (ETNA), located between the well-ventilated subtropical gyre and the equatorial oxygen maximum, is composed of a deep OMZ at about 400 m depth with its core region centred at about 20° W, 10° N and a shallow OMZ at about 100 m depth with lowest oxygen concentrations in proximity to the coastal upwelling region off Mauritania and Senegal. The oxygen budget of the deep OMZ is given by oxygen consumption mainly balanced by the oxygen supply due to meridional eddy fluxes (about 60%) and vertical mixing (about 20%, locally up to 30%). Advection by zonal jets is crucial for the establishment of the equatorial oxygen maximum. In the latitude range of the deep OMZ, it dominates the oxygen supply in the upper 300 to 400 m and generates the intermediate oxygen maximum between deep and shallow OMZs. Water mass ages from transient tracers indicate substantially older water masses in the core of the deep OMZ (about 120-180 years) compared to regions north and south of it. The deoxygenation of the ETNA OMZ during recent decades suggests a substantial imbalance in the oxygen budget: about 10% of the oxygen consumption during that period was not balanced by ventilation. Long-term oxygen observations show variability on interannual, decadal and multidecadal time scales that can partly be attributed to circulation changes. In comparison to the ETNA OMZ the eastern tropical South Pacific OMZ shows a similar structure including an equatorial oxygen maximum driven by zonal advection, but overall much lower oxygen concentrations approaching zero in extended regions. As the shape of the OMZs is set by ocean circulation, the widespread misrepresentation of the intermediate circulation in ocean circulation models substantially contributes to their oxygen bias, which might have significant impacts on predictions of future oxygen levels.