Oliver Huhn

Validity limits of carbon tetrachloride as an ocean tracer

We investigate non-conservative behavior of carbon tetrachloride (CCl4) in the ocean by evaluating concurrent data of this tracer and of the chlorofluorocarbon CFC-12 from three zonal sections in the South Atlantic (∼30°S, 19°S, 11°S; METEOR cruises M22/5, M15/3, M28/1) and from two sections in the western Weddell Sea (POLARSTERN cruises ANT XIII/4, ANT XV/4). The issue is of interest biogeochemically and for uses of CCl4 as a transient ocean tracer. For the South Atlantic a simple model is employed that simulates the meridional tracer transfer into the Central Water and Antarctic Intermediate Water from their southerly outcrops. From a joint fit for the three sections we deduce a CCl4 depletion rate of approximately 22% per year for temperatures exceeding 13°C, which confirms a previous estimate and exceeds rates due to hydrolysis by up to about 50-fold. A tracer utility of CCl4 in warm ocean waters thus hardly exists. However, below ∼13°C the decomposition rates decrease sharply, and they become negligibly small below about 3°C (rate <0.1% per year, compatible with rates due to hydrolysis). In the Weddell Sea we do not find positive evidence of a CCl4 destruction at depth (upper limit 1% per year), in keeping with the South Atlantic result. In the western Weddell Sea deep waters we deduce a apparent CCl4/CFC-12 ratio age of about 30 years. We confirm a previous claim of a CCl4 deficiency in newly formed Weddell Sea deep and bottom waters, which we deduce to amount to approximately 32% relative to CFC-12. We ascribe this deficiency to CCl4 loss within the ventilated source waters (possibly due to interaction with sea or shelf ice), combined with a slower gas transfer from the atmosphere into the upper waters (contribution ∼12%). It is argued that CCl4 deficiencies relative to more stable tracers should be common in newly formed deep and bottom waters, and that assessing such initial deficiencies is a prerequisite for using CCl4 as a tracer. An open question is a CCl4 instability at reduced oxygen concentrations, although the critical oxygen level appears to be lower than reported previously (Tanhua et al., Mar. Chem. 54 (1996) 159). Moreover, temperature might only be a proxy for the real agent that governs CCl4 destruction. The actual mechanism of decomposition remains unknown, but judging from an Arrhenius plot a first-order chemical reaction can be excluded. It is estimated that the ocean contributes roughly 8% to the total environmental destruction of CCl4.

Environmental information for a marine ecosystem research approach for the northern Antarctic Peninsula (RV Polarstern expedition PS81, ANT-XXIX/3)

During the austral summer expedition PS81, ANT-XXIX/3 with the German research ice breaker Polarstern in 2013, research was carried out to investigate the role of environmental factors on the distribution of benthic communities and marine mammal and krill densities around the northern tip of the Antarctic Peninsula. For these studies collated in this special issue and studies in this area, we present a collection of environmental parameters with probable influence on the marine ecosystems around the Antarctic Peninsula.

Water masses in the Bransfield Strait and adjacent seas, austral summer 2013

The Bransfield Strait is a semi-enclosed sea located in the northern part of the West Antarctic Peninsula region, which is subject to strong climatic changes. The bathymetry is complex and comprises three basins that are separated from each other by shallow sills. Oceanographic measurements of the Bransfield Strait region are available since the first half of the twentieth century. In this study, hydrographic data from the ANT-XXIX/3 expedition of RV Polarstern in 2013 are presented to describe the actual physical state of the art, particularly for biological work done during that cruise. The general hydrographic situation of the Bransfield Strait in 2013 is found to be similar to observations from the early twentieth century. The Bransfield Strait’s water masses are modified versions of the water masses from the adjacent seas. The different water masses within the Bransfield Strait are separated by two fronts, the so-called Bransfield and Peninsula Front. While the Bransfield Front is most pronounced in the central and southwestern Bransfield Strait, the Peninsula Front can be identified from the northeastern to the central part of the study domain. Based on an analysis of water mass properties around the Antarctic Peninsula and close to the Antarctic Sound, a notable inflow of Shelf Water from the Weddell Sea through the Antarctic Sound appears unlikely.

Meteorology and oceanography of the Atlantic sector of the Southern Ocean—a review of German achievements from the last decade

In the early 1980s, Germany started a new era of modern Antarctic research. The Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) was founded and important research platforms such as the German permanent station in Antarctica, today called Neumayer III, and the research icebreaker Polarstern were installed. The research primarily focused on the Atlantic sector of the Southern Ocean. In parallel, the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) started a priority program ‘Antarctic Research’ (since 2003 called SPP-1158) to foster and intensify the cooperation between scientists from different German universities and the AWI as well as other institutes involved in polar research. Here, we review the main findings in meteorology and oceanography of the last decade, funded by the priority program. The paper presents field observations and modelling efforts, extending from the stratosphere to the deep ocean. The research spans a large range of temporal and spatial scales, including the interaction of both climate components. In particular, radiative processes, the interaction of the changing ozone layer with large-scale atmospheric circulations, and changes in the sea ice cover are discussed. Climate and weather forecast models provide an insight into the water cycle and the climate change signals associated with synoptic cyclones. Investigations of the atmospheric boundary layer focus on the interaction between atmosphere, sea ice and ocean in the vicinity of polynyas and leads. The chapters dedicated to polar oceanography review the interaction between the ocean and ice shelves with regard to the freshwater input and discuss the changes in water mass characteristics, ventilation and formation rates, crucial for the deepest limb of the global, climate-relevant meridional overturning circulation. They also highlight the associated storage of anthropogenic carbon as well as the cycling of carbon, nutrients and trace metals in the ocean with special emphasis on the Weddell Sea.

Basal Melt and Freezing Rates From First Noble Gas Samples Beneath an Ice Shelf

A climatically-induced acceleration in ocean-driven melting of Antarctic ice shelves would have consequences for both the discharge of continental ice into the ocean and thus global sea level, and for the formation of Antarctic Bottom Water and the oceanic meridional overturning circulation. Using a novel gas-tight in-situ water sampler, noble gas samples have been collected from six locations beneath the Filchner Ice Shelf, the first such samples from beneath an Antarctic Ice shelf. Helium and neon are uniquely suited as tracers of glacial meltwater in the ocean. Basal meltwater fractions range from 3.6% near the ice shelf base to 0.5% near the sea floor, with distinct regional differences. We estimate an average basal melt rate for the Filchner-Ronne Ice Shelf of 177 ± 95 Gt/year, independently confirming previous results. We calculate that up to 2.7% of the meltwater has been refrozen, and we identify a local source of crustal helium.