ALBERT

Description: Here we present a comprehensive attempt to correlate aragonitic Na∕Ca ratios from Desmophyllum pertusum (formerly known as Lophelia pertusa), Madrepora oculata and a caryophylliid cold-water coral (CWC) species with different seawater parameters such as temperature, salinity and pH. Living CWC specimens were collected from 16 different locations and analyzed for their Na∕Ca ratios using solution-based inductively coupled plasma-optical emission spectrometry (ICP-OES) measurements. The results reveal no apparent correlation with salinity (30.1–40.57 g kg−1) but a significant inverse correlation with temperature (−0.31±0.04  mmolmol−1∘C−1). Other marine aragonitic organisms such as Mytilus edulis (inner aragonitic shell portion) and Porites sp. exhibit similar results highlighting the consistency of the calculated CWC regressions. Corresponding Na∕Mg ratios show a similar temperature sensitivity to Na∕Ca ratios, but the combination of two ratios appears to reduce the impact of vital effects and domain-dependent geochemical variation. The high degree of scatter and elemental heterogeneities between the different skeletal features in both Na∕Ca and Na∕Mg, however, limit the use of these ratios as a proxy and/or make a high number of samples necessary. Additionally, we explore two models to explain the observed temperature sensitivity of Na∕Ca ratios for an open and semi-enclosed calcifying space based on temperature-sensitive Na- and Ca-pumping enzymes and transport proteins that change the composition of the calcifying fluid and consequently the skeletal Na∕Ca ratio.

Description: The impact of the positive and negative phases of the Pacific Decadal Oscillation (PDO) on the extension of the poorly oxygenated regions of the eastern Pacific Ocean was assessed using a coupled ocean circulation–biogeochemical model. We show that during a “typical” PDO-positive phase the volume of the suboxic regions expands by 7 % over 50 years due to a slowdown of the large-scale circulation related to the decrease in the intensity of the trade winds. Changes in oxygen levels are mostly controlled by advective processes between 10∘ N and 10∘ S, whereas diffusive processes are dominant poleward of 10∘: in a “typical” PDO-positive phase the sluggish equatorial current system provides less oxygen to the eastern equatorial part of the basin while the oxygen transport by diffusive processes significantly decreases south of 10∘ S. The suboxic region located north of 10∘ N displays less sensitivity to the phase of the PDO as the local upwelling-related processes play a dominant role compared to the large-scale circulation in setting the oxygen concentration. Our study suggests that the prevailing PDO-positive conditions since 1975 may explain a significant part of the current deoxygenation occurring in the eastern Pacific Ocean.

Description: The Mediterranean Sea is considered a "hot spot" for climate change, being characterized by oligotrophic to ultra-oligotrophic waters and rapidly increasing seasurface temperature and changing carbonate chemistry. Coccolithophores are considered a dominant phytoplankton group in these waters. As marine calcifying organisms they are expected to respond to the ongoing changes in seawater carbonate chemistry. We provide here a description of the springtime coccolithophore distribution in the Mediterranean Sea and relate this to a broad set of in situ-measured environmental variables. Samples were taken during the R/V Meteor (M84/3) oceanographic cruise in April 2011, between 0 and 100 m water depth from 28 stations. Total diatom and silicoflagellate cell concentrations are also presented. Our results highlight the importance of seawater carbonate chemistry, especially [CO32−] but also [PO43−] in unraveling the distribution of heterococcolithophores, the most abundant coccolithophore life phase. Holo- and heterococcolithophores respond differently to environmental factors. For instance, changes in heterococcolithophore assemblages were best linked to the combination of [CO32−], pH, and salinity (ρ = 0.57), although salinity might be not functionally related to coccolithophore assemblage distribution. Holococcolithophores, on the other hand, showed higher abundances and species diversity in oligotrophic areas (best fit, ρ = 0.32 for nutrients), thriving in nutrient-depleted waters. Clustering of heterococcolithophores revealed three groups of species sharing more than 65% similarities. These clusters could be assigned to the eastern and western basins and deeper layers (below 50 m), respectively. In addition, the species Gephyrocapsa oceanica, G. muellerae, and Emiliania huxleyi morphotype B/C are spatially distributed together and trace the influx of Atlantic waters into the Mediterranean Sea. The results of the present work emphasize the importance of considering holo- and heterococcolithophores separately when analyzing changes in species assemblages and diversity. Our findings suggest that coccolithophores are a main phytoplankton group in the entire Mediterranean Sea and can dominate over siliceous phytoplankton. They have life stages that are expected to respond differently to the variability in seawater carbonate chemistry and nutrient concentrations.

Description: Climate and marine biogeochemistry changes over the Holocene are investigated based on transient global climate and biogeochemistry model simulations over the last 9500 years. The simulations are forced by accelerated and non-accelerated orbital parameters, respectively, and atmospheric PCO2, CH4, and N2O. The analysis focusses on key climatic parameters of relevance to the marine biogeochemistry, and on the physical and biogeochemical processes that drive atmosphere-ocean carbon fluxes and changes in the oxygen minimum zones (OMZs). The simulated global mean ocean temperature is characterized by a mid-Holocene cooling and a late Holocene warming, a common feature among Holocene climate simulations which, however, contradicts a proxy-derived mid-Holocene climate optimum. As the most significant result, and only in the non-accelerated simulation, we find a substantial increase in volume of the OMZ in the eastern equatorial Pacific (EEP) continuing into the late Holocene. The concurrent increase in apparent oxygen utilization (AOU) and age of the water mass within the EEP OMZ can be attributed to a weakening of the deep northward inflow into the Pacific. This results in a large-scale mid-to-late Holocene increase in AOU in most of the Pacific and hence the source regions of the EEP OMZ waters. The simulated expansion of the EEP OMZ raises the question of whether the deoxygenation that has been observed over the last 5 decades could be a - perhaps accelerated - continuation of an orbitally driven decline in oxygen. Changes in global mean biological production and export of detritus remain of the order of 10 %, with generally lower values in the mid-Holocene. The simulated atmosphere-ocean CO2 flux would result in atmospheric pCO2 changes of similar magnitudes to those observed for the Holocene, but with different timing. More technically, as the increase in EEP OMZ volume can only be simulated with the non-accelerated model simulation, non-accelerated model simulations are required for an analysis of the marine biogeochemistry in the Holocene. Notably, the long control experiment also displays similar magnitude variability to the transient experiment for some parameters. This indicates that also long control runs are required when investigating Holocene climate and marine biogeochemistry, and that some of the Holocene variations could be attributed to internal variability of the atmosphere-ocean system.

Description: Parameterizations of surface ocean isoprene concentrations are numerous, despite the lack of source/sink process understanding. Here we present isoprene and related field measurements in the mixed layer from the Indian Ocean and the eastern Pacific Ocean to investigate the production and consumption rates in two contrasting regions, namely oligotrophic open ocean and the coastal upwelling region. Our data show that the ability of different phytoplankton functional types (PFTs) to produce isoprene seems to be mainly influenced by light, ocean temperature, and salinity. Our field measurements also demonstrate that nutrient availability seems to have a direct influence on the isoprene production. With the help of pigment data, we calculate in-field isoprene production rates for different PFTs under varying biogeochemical and physical conditions. Using these new calculated production rates, we demonstrate that an additional significant and variable loss, besides a known chemical loss and a loss due to air–sea gas exchange, is needed to explain the measured isoprene concentration. We hypothesize that this loss, with a lifetime for isoprene between 10 and 100 days depending on the ocean region, is potentially due to degradation or consumption by bacteria.

Description: The Surface Ocean CO2 Atlas (SOCAT) is a synthesis of quality-controlled fCO2 (fugacity of carbon dioxide) values for the global surface oceans and coastal seas with regular updates. Version 3 of SOCAT has 14.7 million fCO2 values from 3646 data sets covering the years 1957 to 2014. This latest version has an additional 4.6 million fCO2 values relative to version 2 and extends the record from 2011 to 2014. Version 3 also significantly increases the data availability for 2005 to 2013. SOCAT has an average of approximately 1.2 million surface water fCO2 values per year for the years 2006 to 2012. Quality and documentation of the data has improved. A new feature is the data set quality control (QC) flag of E for data from alternative sensors and platforms. The accuracy of surface water fCO2 has been defined for all data set QC flags. Automated range checking has been carried out for all data sets during their upload into SOCAT. The upgrade of the interactive Data Set Viewer (previously known as the Cruise Data Viewer) allows better interrogation of the SOCAT data collection and rapid creation of high-quality figures for scientific presentations. Automated data upload has been launched for version 4 and will enable more frequent SOCAT releases in the future. High-profile scientific applications of SOCAT include quantification of the ocean sink for atmospheric carbon dioxide and its long-term variation, detection of ocean acidification, as well as evaluation of coupled-climate and ocean-only biogeochemical models. Users of SOCAT data products are urged to acknowledge the contribution of data providers, as stated in the SOCAT Fair Data Use Statement. This ESSD (Earth System Science Data) "living data" publication documents the methods and data sets used for the assembly of this new version of the SOCAT data collection and compares these with those used for earlier versions of the data collection (Pfeil et al., 2013; Sabine et al., 2013; Bakker et al., 2014). Individual data set files, included in the synthesis product, can be downloaded here: doi:10.1594/PANGAEA.849770. The gridded products are available here: doi:10.3334/CDIAC/OTG.SOCAT_V3_GRID.

Description: Recent observations in the eastern tropical South Pacific (ETSP) have shown the key role of meso- and submesoscale processes (e.g. eddies) in shaping its hydrographic and biogeochemical properties. Off Peru, elevated primary production from coastal upwelling in combination with sluggish ventilation of subsurface waters fuels a prominent oxygen minimum zone (OMZ). Given that nitrous oxide (N2O) production–consumption processes in the water column are sensitive to oxygen (O2) concentrations, the ETSP is a region of particular interest to investigate its source–sink dynamics. To date, no detailed surveys linking mesoscale processes and N2O distributions as well as their relevance to nitrogen (N) cycling are available. In this study, we present the first measurements of N2O across three mesoscale eddies (two mode water or anticyclonic and one cyclonic) which were identified, tracked, and sampled during two surveys carried out in the ETSP in November–December 2012. A two-peak structure was observed for N2O, wherein the two maxima coincide with the upper and lower boundaries of the OMZ, indicating active nitrification and partial denitrification. This was further supported by the abundances of the key gene for nitrification, ammonium monooxygenase (amoA), and the gene marker for N2O production during denitrification, nitrite reductase (nirS). Conversely, we found strong N2O depletion in the core of the OMZ (O2 〈 5 μmol/L) to be consistent with nitrite (NO2-) accumulation and low levels of nitrate (NO3-), thus suggesting active denitrification. N2O depletion within the OMZ’s core was substantially higher in the centre of mode water eddies, supporting the view that eddy activity enhances N-loss processes off Peru, in particular near the shelf break where nutrient-rich, productive waters from upwelling are trapped before being transported offshore. Analysis of eddies during their propagation towards the open ocean showed that, in general, “ageing” of mesoscale eddies tends to decrease N2O concentrations through the water column in response to the reduced supply of material to fuel N loss, although the hydrographic variability might also significantly impact the pace of the production–consumption pathways for N2O. Our results evidence the relevance of mode water eddies for N2O distribution, thereby improving our understanding of the N-cycling processes, which are of crucial importance in times of climate change and ocean deoxygenation.

Description: Repeat shipboard and multi-year moored observations obtained in the oxygen minimum zone (OMZ) of the eastern tropical North Atlantic (ETNA) were used to study the decadal change in oxygen for the period 2006–2015. Along 23° W between 6 and 14° N, oxygen decreased with a rate of −5.9 ± 3.5 µmol kg−1 decade−1 within the depth covering the deep oxycline (200–400 m), while below the OMZ core (400–1000 m) oxygen increased by 4.0 ± 1.6 µmol kg−1 decade−1 on average. The inclusion of these decadal oxygen trends in the recently estimated oxygen budget for the ETNA OMZ suggests a weakened ventilation of the upper 400 m, whereas the ventilation strengthened homogeneously below 400 m. The changed ventilation resulted in a shoaling of the ETNA OMZ of −0.03 ± 0.02 kg m−3 decade−1 in density space, which was only partly compensated by a deepening of isopycnal surfaces, thus pointing to a shoaling of the OMZ in depth space as well (−22 ± 17 m decade−1). Based on the improved oxygen budget, possible causes for the changed ventilation are analyzed and discussed. Largely ruling out other ventilation processes, the zonal advective oxygen supply stands out as the most probable budget term responsible for the decadal oxygen changes.

Description: During the Last Interglacial (LIG, ∼130–115kiloyears (kyr) before present (BP)), the northern high latitudes were characterized by higher temperatures than those of the late Holocene and a lower Greenland Ice Sheet (GIS). However, the impact of a reduced GIS on the global climate has not yet been well constrained. In this study, we quantify the contribution of the GIS to LIG warmth by performing various sensitivity studies based on equilibrium simulations, employing the Community Earth System Models (COSMOS), with a focus on height and extent of the GIS. We present the first study on the effects of a reduction in the GIS on the surface temperature (TS) on a global scale and separate the contribution of astronomical forcing and changes in GIS to LIG warmth. The strong Northern Hemisphere summer warming of approximately 2°C (with respect to pre-industrial) is mainly caused by increased summer insolation. Reducing the height by  ∼ 1300m and the extent of the GIS does not have a strong influence during summer, leading to an additional global warming of only +0.24°C compared to the purely insolation-driven LIG. The effect of a reduction in the GIS is, however, strongest during local winter, with up to +5°C regional warming and with an increase in global average temperature of +0.48°C. In order to evaluate the performance of our LIG simulations, we additionally compare the simulated TS anomalies with marine and terrestrial proxy-based LIG temperature anomalies derived from three different proxy data compilations. Our model results are in good agreement with proxy records with respect to the warming pattern but underestimate the magnitude of temperature change when compared to reconstructions, suggesting a potential misinterpretation of the proxy records or deficits in our model. However, we are able to partly reduce the mismatch between model and data by additionally taking into account the potential seasonal bias of the proxy record and/or the uncertainties in the dating of the proxy records for the LIG thermal maximum. The seasonal bias and the uncertainty of the timing are estimated from new transient model simulations covering the whole LIG. The model–data comparison improves for proxies that represent annual mean temperatures when the GIS is reduced and when we take the local thermal maximum during the LIG (130–120kyrBP) into account. For proxy data that represent summer temperatures, changes in the GIS are of minor importance for sea surface temperatures. However, the annual mean and summer temperature change over Greenland in the reduced GIS simulations seems to be overestimated as compared to the local ice core data, which could be related to the interpretation of the recorder system and/or the assumptions of GIS reduction. Thus, the question regarding the real size of the GIS during the LIG has yet to be answered.

Description: PALADYN is presented; it is a new comprehensive and computationally efficient land surface–vegetation–carbon cycle model designed to be used in Earth system models of intermediate complexity for long-term simulations and paleoclimate studies. The model treats in a consistent manner the interaction between atmosphere, terrestrial vegetation and soil through the fluxes of energy, water and carbon. Energy, water and carbon are conserved. PALADYN explicitly treats permafrost, both in physical processes and as an important carbon pool. It distinguishes nine surface types: five different vegetation types, bare soil, land ice, lake and ocean shelf. Including the ocean shelf allows the treatment of continuous changes in sea level and shelf area associated with glacial cycles. Over each surface type, the model solves the surface energy balance and computes the fluxes of sensible, latent and ground heat and upward shortwave and longwave radiation. The model includes a single snow layer. Vegetation and bare soil share a single soil column. The soil is vertically discretized into five layers where prognostic equations for temperature, water and carbon are consistently solved. Phase changes of water in the soil are explicitly considered. A surface hydrology module computes precipitation interception by vegetation, surface runoff and soil infiltration. The soil water equation is based on Darcy's law. Given soil water content, the wetland fraction is computed based on a topographic index. The temperature profile is also computed in the upper part of ice sheets and in the ocean shelf soil. Photosynthesis is computed using a light use efficiency model. Carbon assimilation by vegetation is coupled to the transpiration of water through stomatal conductance. PALADYN includes a dynamic vegetation module with five plant functional types competing for the grid cell share with their respective net primary productivity. PALADYN distinguishes between mineral soil carbon, peat carbon, buried carbon and shelf carbon. Each soil carbon type has its own soil carbon pools generally represented by a litter, a fast and a slow carbon pool in each soil layer. Carbon can be redistributed between the layers by vertical diffusion and advection. For the vegetated macro surface type, decomposition is a function of soil temperature and soil moisture. Carbon in permanently frozen layers is assigned a long turnover time which effectively locks carbon in permafrost. Carbon buried below ice sheets and on flooded ocean shelves is treated differently. The model also includes a dynamic peat module. PALADYN includes carbon isotopes 13C and 14C, which are tracked through all carbon pools. Isotopic discrimination is modelled only during photosynthesis. A simple methane module is implemented to represent methane emissions from anaerobic carbon decomposition in wetlands (including peatlands) and flooded ocean shelf. The model description is accompanied by a thorough model evaluation in offline mode for the present day and the historical period