Physical Oceanography in the tropical Atlantic oxygen minimum zone

The replenishment of consumed oxygen in the open ocean oxygen minimum zone (OMZ) off northwest Africa is accomplished by oxygen transport across and along density surfaces, i.e. diapycnal and isopycnal oxygen supply. Here the diapycnal oxygen supply is investigated using a large observational set of oxygen profiles and diapycnal mixing data from years 2008 to 2010. Diapycnal mixing is inferred from different sources: (i) a large-scale tracer release experiment, (ii) microstructure profiles, and (iii) shipboard?acoustic current measurements plus density profiles. From these measurements, the average diapycnal diffusivity in the studied depth interval from 150 to 500m is estimated to be 1×10**-5 m2 s**-1, with lower and upper 95% confidence limits of 0.8×10**-5 m2 s**-1 and 1.4×10**-5 m2 s**-1. Diapycnal diffusivity in this depth range is predominantly caused by turbulence, and shows no significant vertical gradient. Diapycnal mixing is found to contribute substantially to the oxygen supply of the OMZ. Within the OMZ core, 1.5 µmol kg**-1 yr**-1 of oxygen is supplied via diapycnal mixing, contributing about one-third of the total demand. This oxygen which is supplied via diapycnal mixing originates from oxygen that has been laterally supplied within the upper CentralWater layer above the OMZ, and within the Antarctic Intermediate Water layer below the OMZ. Due to the existence of a separate shallow oxygen minimum at about 100m depth throughout most of the study area, there is no net vertical oxygen flux from the surface layer into the Central Water layer. Thus all oxygen supply of the OMZ is associated with remote pathways.

Hydrostatic pressure affects aggregate transformation and organic carbon transport: in and beyond the twilight zone: Experiment December 2010

This work aimed to explore evaluated the effects of the increased of hydrostatic pressure on a defined bacterial community on aggregates formed from an axenic culture of marine diatoms by simulating sedimentation to the deep sea by increase of hydrostatic pressure up to 30 bar (equivalent to 3000 m water depth) against control at ambient surface pressure. Our hypothesis was that microbial colonization and community composition and thus microbial OM turnover is greatly affected by changes in hydrostatic pressure during sinking to the deep ocean.

Porewater and particulate geochemistry during Maria S. Merian cruise MSM17/4

We present sedimentary geochemical data and in situ benthic flux measurements of dissolved inorganic nitrogen (DIN: NO3-, NO2-, NH4+) and oxygen (O2) from 7 sites with variable sand content along 18°N offshore Mauritania (NW Africa). Bottom water O2 concentrations at the shallowest station were hypoxic (42 µM) and increased to 125 µM at the deepest site (1113 m). Total oxygen uptake rates were highest on the shelf (-10.3 mmol O2 /m2 d) and decreased quasi-exponentially with water depth to -3.2 mmol O2 /m2 d. Average denitrification rates estimated from a flux balance decreased with water depth from 2.2 to 0.2 mmol N /m2 d. Overall, the sediments acted as net sink for DIN. Observed increases in delta 15NNO3 and delta 18ONO3 in the benthic chamber deployed on the shelf, characterized by muddy sand, were used to calculate apparent benthic nitrate fractionation factors of 8.0 pro mille (15epsilon app) and 14.1 pro mille (18epsilon app). Measurements of delta 15NNO2 further demonstrated that the sediments acted as a source of 15N depleted NO2-. These observations were analyzed using an isotope box model that considered denitrification and nitrification of NH4+ and NO2-. The principal findings were that (i) net benthic 14N/15N fractionation (epsilon DEN) was 12.9 ± 1.7pro mille, (ii) inverse fractionation during nitrite oxidation leads to an efflux of isotopically light NO2- (-22 ± 1.9 pro mille), and (iii) direct coupling between nitrification and denitrification in the sediment is negligible. Previously reported epsilon DEN for fine-grained sediments are much lower (4-8 pro mille). We speculate that high benthic nitrate fractionation is driven by a combination of enhanced porewater-seawater exchange in permeable sediments and the hypoxic, high productivity environment. Although not without uncertainties, the results presented could have important implications for understanding the current state of the marine N cycle.

Fluid compositions and mineralogy of precipitates from Mid Atlantic Ridge hydrothermal vents at 4°48'S

The effect of volcanic activity on submarine hydrothermal systems has been well documented along fast- and intermediate-spreading centers but not from slow-spreading ridges. Indeed, volcanic eruptions are expected to be rare on slow-spreading axes. Here we report the presence of hydrothermal venting associated with extremely fresh lava flows at an elevated, apparently magmatically robust segment center on the slow-spreading southern Mid-Atlantic Ridge near 5°S. Three high-temperature vent fields have been recognized so far over a strike length of less than 2 km with two fields venting phase-separated, vapor-type fluids. Exit temperatures at one of the fields reach up to 407°C, at conditions of the critical point of seawater, the highest temperatures ever recorded from the seafloor. Fluid and vent field characteristics show a large variability between the vent fields, a variation that is not expected within such a limited area. We conclude from mineralogical investigations of hydrothermal precipitates that vent-fluid compositions have evolved recently from relatively oxidizing to more reducing conditions, a shift that could also be related to renewed magmatic activity in the area. Current high exit temperatures, reducing conditions, low silica contents, and high hydrogen contents in the fluids of two vent sites are consistent with a shallow magmatic source, probably related to a young volcanic eruption event nearby, in which basaltic magma is actively crystallizing. This is the first reported evidence for direct magmatic-hydrothermal interaction on a slow-spreading mid-ocean ridge.

Paleotemperature reconstruction for the past 0.55 Ma of the Subantarctic zone

Estimates of summer sea surface temperatures (SSSTs) derived from planktic foraminiferal associations using the Modern Analog Technique and combined with isotopic analyses and determination of ice-rafted debris, mirror the Pleistocene evolution of the planktic Subantarctic surface waters in the Atlantic Ocean. The SSSTs indicate that the isotherms that define the modern polar front zone and Subantarctic front, were located at more northerly latitudes (up to 7°) during most of the investigated period, which covers the past 550 kyr. Exceptions are during climatic optima in the early Holocene, at marine isotope stages (MIS) 5.5, 7.1, 7.5, 9.3, and presumably during MIS 11.3 when SSSTs exceeded modern values by 1 -5°C. The close similarity between the SSST and the Vostok temperature indicates strong regional temperature correlation. Both records show that MIS 9.3 was the warmest period during the last 420 kyr whereas SSSTs obtained for MIS 11.3 are overestimated due to strong carbonate dissolution. Spectral analysis corroborates that the initiation of warming in southern high latitudes heralds the start of deglaciation on the Northern Hemisphere.