Alfred Wegener Institute for Polar and Marine Research
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The mixed layer depth in Ocean Model Intercomparison Project (OMIP) high resolution models
Get PDFThe ocean mixed layer is the interface between the ocean interior and the atmosphere or sea ice, and plays a key role in climate variability. Numerical models used in climate studies should therefore have a good representation of the mixed layer, especially its depth (MLD). Here we use simulations from the Ocean Model Intercomparison Project (OMIP), which have been forced by a common atmospheric state, to assess the realism of the simulated MLDs. For model validation, an updated MLD dataset has been computed from observations using the fixed density threshold recommended by the OMIP protocol. We evaluate the influence of horizontal resolution by using six pairs of simulations, non-eddying (typically 1&#176; resolution) and eddy-rich (1/10&#176; to 1/16&#176; resolution). In winter, low resolution models exhibit large biases in the deep water formation regions. These biases are reduced in eddy-rich models but not uniformly across models and regions. The improvement is most noticeable in the mode water formation regions of the northern hemisphere, where the eddy-rich models produce a more robust MLD and deep biases are reduced. The Southern Ocean offers a more contrasted view, with biases of either sign remaining at high resolution. In eddy-rich models, mesoscale eddies control the spatial variability of MLD in winter. Contrary to an hypothesis that the deepening of the MLD in anticyclones would make the MLD deeper globally, eddy-rich models tend to have a shallower MLD in the zonal mean. In summer, a deep MLD bias is found in all the non-eddying models north of the equator; this bias is greatly reduced at high resolution. In addition, our study highlights the sensitivity of the MLD computation to choice of a reference level and the spatio-temporal sampling, which motivates new recommendations for MLD computation in future model intercomparison projects.</jats:p
Southern Hemisphere atmospheric history of carbon monoxide over the late Holocene reconstructed from multiple Antarctic ice archives
Get PDFCarbon monoxide (CO) is a naturally occurring atmospheric trace gas, a regulated pollutant, and one of the main components determining the oxidative capacity of the atmosphere. Evaluating climate-chemistry models under different conditions than today and constraining past CO sources requires a reliable record of atmospheric CO mixing ratios ([CO]) that includes data since preindustrial times. Here, we report the first continuous record of atmospheric [CO] for Southern Hemisphere (SH) high latitudes over the past 3 millennia. Our continuous record is a composite of three high-resolution Antarctic ice core gas records and firn air measurements from seven Antarctic locations. The ice core gas [CO] records were measured by continuous flow analysis (CFA), using an optical feedback cavity-enhanced absorption spectrometer (OF-CEAS), achieving excellent external precision (2.8-8.8gppb; 2σ) and consistently low blanks (ranging from 4.1±1.2 to 7.4±1.4gppb), thus enabling paleo-Atmospheric interpretations. Six new firn air [CO] Antarctic datasets collected between 1993 and 2016gCE at the DE08-2, DSSW19K, DSSW20K, South Pole, Aurora Basin North (ABN), and Lock-In sites (and one previously published firn CO dataset at Berkner) were used to reconstruct the atmospheric history of CO from g1/41897gCE, using inverse modeling that incorporates the influence of gas transport in firn. Excellent consistency was observed between the youngest ice core gas [CO] and the [CO] from the base of the firn and between the recent firn [CO] and atmospheric [CO] measurements at Mawson station (eastern Antarctica), yielding a consistent and contiguous record of CO across these different archives. Our Antarctic [CO] record is relatively stable from-835 to 1500gCE, with mixing ratios within a 30-45gppb range (2σ). There is a g1/45gppb decrease in [CO] to a minimum at around 1700gCE during the Little Ice Age. CO mixing ratios then increase over time to reach a maximum of g1/454gppb by g1/41985gCE. Most of the industrial period [CO] growth occurred between about 1940 to 1985gCE, after which there was an overall [CO] decrease, as observed in Greenland firn air and later at atmospheric monitoring sites and attributed partly to reduced CO emissions from combustion sources. Our Antarctic ice core gas CO observations differ from previously published records in two key aspects. First, our mixing ratios are significantly lower than reported previously, suggesting that previous studies underestimated blank contributions. Second, our new CO record does not show a maximum in the late 1800s. The absence of a [CO] peak around the turn of the century argues against there being a peak in Southern Hemisphere biomass burning at this time, which is in agreement with (i) other paleofire proxies such as ethane or acetylene and (ii) conclusions reached by paleofire modeling. The combined ice core and firn air [CO] history, spanning-835 to 1992gCE, extended to the present by the Mawson atmospheric record, provides a useful benchmark for future atmospheric chemistry modeling studies
Evolution of complex giant seafloor depressions at the northern Argentine continental margin (SW Atlantic Ocean) under the influence of a dynamic bottom current regime
Get PDFSeafloor depressions (SD) are features commonly observed on the ocean floor. They often occur as circular, small-sized (up to 10 s of m) incisions caused by fluid expulsion. Larger depressions (100s m to km) are considerably less abundant, and their origin and development have been scarcely studied. This study investigated two giant morphological depressions (>5 km) using recently acquired multibeam bathymetry and backscatter, sediment echosounder, and high-resolution seismic data. An arc-shaped (SD-N) and a sub-circular depression (SD-S) are located on the Ewing Terrace at the Argentine Continental Margin north and south of the Mar del Plata Canyon, respectively. The study area is influenced by the Brazil-Malvinas Confluence, where major counterflowing ocean currents affect sedimentation, and northward flowing currents form a large contourite depositional system. Using an existing seismo-stratigraphy, the onset of SD-N was dated to the middle Miocene (∼15–17 Ma), whereas SD-S started developing at the Miocene/Pliocene boundary (∼6 Ma). Acoustic anomalies indicate the presence of gas and diffuse upward fluid migration, and therefore seafloor seepage is proposed as the initial mechanism for SD-S, whereas we consider a structural control for SD-N to be most likely. Initial depressions were reworked and maintained by strong and variable bottom currents, resulting in prograding clinoform reflection patterns (SD-N) or leading to the build-up of extensive cut-and-fill structures (SD-S). Altogether, this study highlights the evolution of two unique and complex seafloor depressions throughout the geologic past under intense and variable bottom current activity in a highly dynamic oceanographic setting
Occurrence and behavioral rhythms of the endangered Acadian redfish (Sebastes fasciatus) in the Sambro Bank (Scotian Shelf)
Get PDFThe genus Sebastes is a morphologically and ecologically diverse genus of rockfish characterized by high longevity, late-maturity and low natural mortality. On the northwest Atlantic continental shelf, the Acadian redfish (Sebastes fasciatus) is the most common rockfish species above 300 m depth. This species has been widely exploited resulting in the depletion or collapse of most of its stocks. Management of long-lived species with intricate life-history characteristics is challenging and requires highly integrated biological and oceanographic monitoring, which allow the identification of environmental drivers and demographic and behavioral trends. The present study uses high-temporal resolution imaging and environmental data, acquired with an autonomous lander deployed for 10-months at the Sambro Bank Sponge Conservation Area (Scotian Shelf) to elucidate S. fasciatus temporal dynamics and behavioral trends in response to near-bed environmental conditions. S. fasciatus, mostly displayed passive locomotion and static behaviors, in common with other shelf-dwelling Sebastes species. Structural complexity provided by sponges positively influenced S. fasciatus presence. Fish used sponges to avoid being dragged by bottom currents. Hydrodynamics appear to act as a synchronizing factor conditioning its swimming behavior. S. fasciatus total counts exhibited a seasonal shift in rhythm’s phase likely reflecting changes in lifestyle requirements. This study provides new insights on S. fasciatus dynamics and behavior. Nonintrusive monitoring approaches, such as the one used in this study, will be key to monitor this threatened species populations. Especially, since it is expected that S. fasciatus will experience distribution shifts to higher latitudes due to future climate stressors
A year of transient tracers (chlorofluorocarbon 12 and sulfur hexafluoride), noble gases (helium and neon), and tritium in the Arctic Ocean from the MOSAiC expedition (2019–2020)
Get PDFTrace gases have demonstrated their strength for oceanographic studies, with applications ranging from the tracking of glacial meltwater plumes to estimates of the abyssal overturning duration. Yet measurements of such passive tracers in the ice-covered Arctic Ocean are sparse. We here present a unique data set of trace gases collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, during which R/V Polarstern drifted along with the Arctic sea ice from the Laptev Sea to Fram Strait, from October 2019 to September 2020. During the expedition, trace gases from anthropogenic origin (chlorofluorocarbon 12 (CFC-12), sulfur hexafluoride (SF6), and tritium) along with noble gases (helium and neon) and their isotopes were collected at a weekly or higher temporal resolution throughout the entire water column (and occasionally in the snow) from the ship and from the ice. We describe the sampling procedures along with their challenges, the analysis methods, and the data sets, and we present case studies in the central Arctic Ocean and Fram Strait to illustrate possible usage for the data along with their robustness. Combined with simultaneous hydrographic measurements, these trace gas data sets can be used for process studies and water mass tracing throughout the Arctic in subsequent analyses. The two data sets can be downloaded via PANGAEA: https://doi.org/10.1594/PANGAEA.961729 (Huhn et al., 2023a) and https://doi.org/10.1594/PANGAEA.961738 (Huhn et al., 2023b)
Core-Log -Seismic Integration for the Cretaceous to Oligocene sequence in the African-Southern Ocean gateway: First results from the Agulhas Plateau (IODP Sites U1579 and U1580)
Get PDFClimate models have identified significant geography-related Cenozoic cooling arising from the opening of Southern Ocean gateways (e.g., Sauermilch et al., 2021). For example, a gradual strengthening of the Antarctic Circumpolar Current (ACC) has been proposed as the primary cause of cooler deep ocean temperatures associated with the transition from the Cretaceous "Supergreenhouse" to the Oligocene icehouse (e.g., Sijp et al., 2014).
IODP Expedition 392 'Agulhas Plateau Cretaceous Climate' drilled four sites in the African-Southern Ocean gateway in 2022 to significantly advance understanding of how temperatures, ocean circulation, and sedimentation patterns evolved as CO2 levels rose and fell and Gondwana broke up (Uenzelmann-Neben et al., 2022). Prior to Exp. 392, seismostratigraphic models developed for the African-Southern Ocean gateway relied solely on age data from remote drilling sites and information from piston cores, gravity cores, and dredge samples. As a result, a high level of uncertainty had to be assumed for reflector age information, particularly for the sparsely sampled pre-Miocene sedimentary column.
We here present preliminary findings from a revised seismostratigraphy based on core-log-seismic integration at Sites U1579 (central Agulhas Plateau) and U1580 (southern Agulhas Plateau), both of which are located at the heart of the African-Southern Ocean gateway. Core density and velocity data were edited, corrected to in-situ conditions, and combined with downhole logging data. Synthetic seismograms provide an accurate traveltime to depth conversion and show an excellent correlation of drilling results with the site survey seismic field records.
A first correlation of the seismic reflection data with the preliminary shipboard bio- and magnetostratigraphy reveals that the published seismostratigraphic models are erroneous and need revisions, for example, a reflector interpreted to represent a lower Eocene sea level highstand on the Agulhas Plateau has now been identified as the Cretaceous/Paleogene (K/Pg) boundary. Further prominent reflectors can be associated with the Eocene-Oligocene transition (EOT), the top and bottom of zeolitic siliciclastic sandstones (Santonian) and intra-basalt reflections, interpreted as sills.
The core-seismic correlation allows tying major changes in other physical properties (e.g., colour reflectance, natural gamma radiation), chemical composition (e.g., major element ratios from XRF core scanning) and sedimentological parameters (e.g., grain size) to the seismic grids, which will aid reconstructions of oceanic circulation changes and magmatism variations in relation to the development of the Agulhas Plateau
Environmental and physiological conditions that led to the rise of calcifying nannoplankton in the Late Triassic
Get PDFThe rise of calcifying nannoplankton in the Late Triassic (237–201 Ma) had profound consequences for the carbonate buffering capacity of the ocean. Research on the appearance of calcification typically focuses on the reasons that made this process successful on a global scale. The underlying environmental and physiological conditions that led to its appearance, therefore, are still obscure. Here we show that the rise of intracellular calcification is intrinsically linked to cellular calcium metabolism and to the prevention of calcium accumulation in the cytoplasm. Using gene tree analysis, we present evidence that the physiological machinery for calcification was already present in non-calcifying ancestor cells. Additionally, by modelling the energy demands for calcium transport in calcifying and non-calcifying cells, we demonstrate that intracellular calcification does not require additional energy investments. Since all eukaryotic cells export calcium across the plasma membrane, our findings indicate that the onset of intracellular calcification in Earth’s history required only the activation of calcium transport proteins during their passage to the plasma membrane. Hence, intracellular calcification could have occurred at any time in the geological past because physiological preconditions were already present, but a combination of favourable environmental factors may have helped calcifying nannoplankton to spread at the beginning of the Mesozoic, a time of catastrophic climate changes and mass extinctions that reshuffled life on Earth
A fossil pollen dataset of China
Get PDFFossil pollen and spore records provide highly creditable proxy data to investigate the past environmental changes such as palaeovegetation and palaeoclimate. Pollen database promotes past environmental studies from local to regional and global scales and from qualitative to quantitative reconstructions. This is of great significance on exploring the interactions among past vegetation, climates and anthropogenic disturbances at large spatial scale and long temporal scale, to better understand the evolution of the earth system. In this paper, a fossil pollen dataset of China is compiled, by synthesizing 372 original or digitized fossil pollen records including 790 pollen taxa in China’s land and ocean during the late-Quaternary (since 50 ka BP). The dataset includes site names, latitude, longitude and altitude, pollen data source, sample type, sediment length or span, sample number of each site, dating method and dating number, age span and reference, as well as the fossil pollen percentage of each sampling site. The pollen data, mostly published from late 1980s to present, are concentrated in vegetation regions of temperate and subtropical forests, temperate grasslands, temperate deserts and alpine vegetation on the Qingzang Plateau. Sample sites are distributed at different altitudes from deep sea to high Qingzang Plateau, but the majority of the sites are located between 0–2 000 m. The dataset comprises of 178 raw pollen records (47.8%) and 194 digitized pollen records (52.2%). Pollen samples are mainly from lake sediment (151 sites), alluvial/fluvial sediment (99 sites), and peat (67 sites), accounting for 85.2% of the total sampling sites. Radiocarbon is the main dating method that accounts for 93.8% of total samples, and most of the sites have 2–10 radiocarbon dating data. Each site has an average number of pollen taxa of 19, with the most sites having 4–30 pollen taxa. The temporal and spatial distribution of representative pollen taxa (Pinus, Quercus, Artemisia and Poaceae) reveals increasing trends both in their distributional range and pollen concentration from the last glacial maximum to Holocene, but such trends have various regional patterns in different parts of China. This fossil pollen dataset is a fruitful work of collection of pollen records in most territory of China that conducted by palynologists from China and overseas during the last half century. It consolidates the valuable and fundamental data that can be potentially utilized to explore the evolution of past environments and their driving mechanism of climate change and human disturbance