ALBERT

Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Mesoscale eddies are frequently observed in the Eastern Tropical North Atlantic (ETNA), yet their effects on the transport and distribution of biogeochemical solutes, and specifically on the production and remineralization of dissolved organic matter (DOM) remain difficult to elucidate. Here, we investigated the submesoscale variability of chromophoric DOM (CDOM) and fluorescent DOM (FDOM) together with microbial production and remineralization processes in two cyclonic eddies (CEs) in the ETNA during summer and winter 2019. One CE, formed near the coast off Mauritania during the post‐upwelling season, was sampled along a ∼900 km zonal corridor between Mauritania and the Cape Verde Islands. The other CE, formed nearby Brava Island, was out of coastal influence. Four fluorescent components were identified with parallel factor analysis, two humic‐like, and two protein‐like components. Humic‐like FDOM components correlated to optode‐based community respiration and were also good indicators of upwelling associated with the Brava Island CE as they correlated to physical parameters (e.g., temperature) and to dissolved inorganic nitrogen. The tryptophan‐like FDOM components correlated with the carbon and nitrogen content of semi‐labile DOM, phytoplankton biomass, community respiration, and bacterial production. Overall, our study revealed that DOM optical properties are suitable for tracing freshly produced organic matter and the transport of remineralized DOM within offshore eddies.〈/p〉

Description: Plain Language Summary: Mesoscale eddies are ubiquitous circulation features in the ocean with horizontal scales on the order of 100 km and lifetimes of days to months. Their swirling motion can cause nutrients from deeper waters to be transported to the surface, stimulating phytoplankton biomass and resulting in the production of dissolved organic matter. However, these effects are difficult to quantify and proxies (biomarkers) are needed to monitor the impact of eddies at high resolution. In this work, we used the optical properties of the dissolved organic matter, especially the fraction capable of fluorescence (FDOM) as biomarker in two cyclonic eddies, one formed in an eastern boundary upwelling system and one formed offshore by winds/Island interaction. We identified four FDOM components, among which an indicator of cyclonic eddy productivity and two indicators of dissolved organic matter recycling, which also tracked nutrient transport in the offshore cyclonic eddy. Our study highlights that continuous FDOM data obtained with sensors could help to follow eddy development and influence on seawater biogeochemistry.〈/p〉

Description: Key Points: 〈list list-type="bullet"〉 〈list-item〉 〈p xml:lang="en"〉Four fluorescent dissolved organic matter (FDOM) components were studied in two cyclonic eddies (CEs) in the Eastern Tropical North Atlantic〈/p〉〈/list-item〉 〈list-item〉 〈p xml:lang="en"〉Tryptophan‐like FDOM was an indicator of the CEs' productivity as it correlated with semi‐labile dissolved organic matter and microbial metabolic activities〈/p〉〈/list-item〉 〈list-item〉 〈p xml:lang="en"〉Humic‐like FDOM was a by‐product of microbial respiration; its distribution within an offshore CE covaried with nutrient upwelling〈/p〉〈/list-item〉 〈/list〉 〈/p〉

Description: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347

Description: Arctic lakes and wetlands contribute a substantial amount of methane to the contemporary atmosphere, yet profound knowledge gaps remain regarding the intensity and climatic control of past methane emissions from this source. In this study, we reconstruct methane turnover and environmental conditions, including estimates of mean annual and summer temperature, from a thermokarst lake (Lake Qalluuraq) on the Arctic Coastal Plain of northern Alaska for the Holocene by using source-specific lipid biomarkers preserved in a radiocarbon-dated sediment core. Our results document a more prominent role for methane in the carbon cycle when the lake basin was an emergent fen habitat between ~12,300 and ~10,000 cal yr BP, a time period closely coinciding with the Holocene Thermal Maximum (HTM) in North Alaska. Enhanced methane turnover was stimulated by relatively warm temperatures, increased moisture, nutrient supply, and primary productivity. After ~10,000 cal yr BP, a thermokarst lake with abundant submerged mosses evolved, and through the mid-Holocene temperatures were approximately 3°C cooler. Under these conditions, organic matter decomposition was attenuated, which facilitated the accumulation of submerged mosses within a shallower Lake Qalluuraq. Reduced methane assimilation into biomass during the mid-Holocene suggests that thermokarst lakes are carbon sinks during cold periods. In the late-Holocene from ~2700 cal yr BP to the most recent time, however, temperatures and carbon deposition rose and methane oxidation intensified, indicating that more rapid organic matter decomposition and enhanced methane production could amplify climate feedback via potential methane emissions in the future.

Description: Archaea occupy an important niche in the global carbon cycle and their lipids are widely used as indicators of environmental conditions in both paleoenvironmental and modern biogeochemical studies. The principal sources of archaeal lipids in marine sediments are benthic archaea, fossil remnants of planktonic archaea, and allochthonous sources such as soils. However, the relative contributions of these sources to the sedimentary lipid pool have not been comprehensively constrained, complicating a mechanistic understanding of archaeal lipid proxies. In order to provide insights into the relative contributions of these sources and identify signals derived from the sedimentary activity, we performed a systematic survey of concentrations and stable carbon isotopic compositions of both core and intact archaeal lipids The sample set consisted of 44 sediment horizons from the Mediterranean and adjacent basins and represented diverse sources of organic matter and depositional conditions.

Description: Auto- and heterotrophic microbial activity (biomass production and respiration) were investigated in a cyclonic eddy that formed off Mauritania along the ∼ 900 km zonal corridor between Mauritania and the Cabo Verde islands in the eastern Tropical North Atlantic during the M156 cruise on the RV Meteor from July 3rd to August 1st 2019. The dataset includes measured and calculated data over the epipelagic layer (0-200 m depth) of 25 stations with 14 of them inside or in the vicinity of a cyclonic eddy. Temperature, salinity, and oxygen were obtained from a Seabird 911 plus CTD system equipped with two independently working sets of temperature–conductivity–oxygen. Seawater samples were collected using 10 L Niskin bottles attached to the CTD Rosette. Ammonium was analysed based on Solórzano (1969, https://doi.org/10.4319/lo.1969.14.5.0799) and nitrate, nitrite, phosphate and silicate were measured photometrically with continuous-flow analysis on an auto-analyser based on Hansen and Koroleff, (1999, https://doi.org/10.1002/9783527613984.ch10). To estimate the fraction of semi-labile dissolved organic carbon, we determined high-molecular-weight (〉1 kDa) dissolved combined carbohydrates (dCCHO) based on Engel and Händel (2011, https://doi.org/10.1093/plankt/fbq122) and dissolved hydrolysable amino acids (dHAA) based on Lindroth and Mopper (1979, https://https://doi.org/10.1021/ac50047a019) and Dittmar et al, (2009). The analysis of DCCHO detected 11 monomers: and the dHAA analysis classified 13 monomers. The calculations for the carbon content of dCCHO and dHAA were based on carbon atoms contained in the identified monomers. The sum of dCCHO and dHAA carbon content is referred to as SL-DOC. Chlorophyll a was measured from photometric analysis based on Evans et al, (1987). Heterotrophic bacteria, photosynthetic bacteria (Prochlorococcus and Synechococcus), and autotrophic pico and nanoplankton (〈20 μm) abundances were measured by flow cytometry. We converted the cell abundance of the different autotrophic pico- and nanoplankton populations into biomass based on Hernández-Hernández et al. (2020, https://doi.org/10.3389/fmars.2020.00667). Extracellular release rates, dissolved-, particulate- and total- primary production rates were determined from 14C incorporation according to Nielsen (1952, https://doi.org/10.1093/icesjms/18.2.117) and Gargas (1975). Community respiration was obtained from optode-based method from incubations by measuring changes in dissolved oxygen over 24–36 h. Bacterial biomass production rates were measured through the incorporation of labelled leucine (3H) using the microcentrifuge method (Kirchman et al., 1985, https://doi.org/10.1128/aem.49.3.599-607.1985; Smith and Azam, 1992, ). Community respiration and bacterial biomass production were converted to rates at 22°C using equations from Regaudie-De-Gioux and Duarte (2012, https://doi.org/10.1029/2010GB003907) and from López-Urrutia and Morán (2007, https://doi.org/10.1890/06-1641) respectively. Community respiration rates were converted to bacteria respiration rates based on Aranguren-Gassis et al, (2012, https://doi.org/10.3354/meps09707). Bacteria carbon demand and growth efficiency were calculated from bacterial production and respiration rates. Dittmar, T., Cherrier, J., and Ludwichowski, K. U.: The analysis of amino acids in seawater, in: Practical guidelines for the analysis of seawater, edited by: Wurl, O., 67–78, CRC Press, Boca Raton, ISBN: 978-1-4200-7306-5, 2009. Evans, C. A., O'Reily, J. E., and Thomas, J. P.: A handbook for measurement of Chl a and primary production, College Station, TX, Texas A and M University, ISBN:9780948277078, 0948277076, 1987. Gargas, E.: A Manual for Phytoplankton Primary Production Studies in the Baltic, The Baltic Marine Biologists, 2, 88 pp. Ed. Gargas E. (Hørsholm, Denmark: Water Quality Institute), 1975. Smith, D. and Azam, F.: A simple, economical method for measuring bacterial protein synthesis rates in seawater using, Mar. Microb. Food Webs, 6, 107–114, 1992.

Description: Reconstructing ocean temperature values is a major target in paleoceanography and climate research. However, most temperature proxies are organism-based and thus suffer from an "ecological bias". Multiproxy approaches can potentially overcome this bias but typically require more investment in time and resources, while being susceptible to errors induced by sample preparation steps necessary before analysis. Three lipid-based temperature proxies are widely used: UK'37 (based on long chain alkenones from phytoplanktonic haptophytes), TEX86 [based on glycerol dialkyl glycerol tetraethers (GDGTs) from pelagic archaea] and LDI (based on long chain diols from phytoplanktonic eustigmatophytes). So far, separate analytical methods, including gas chromatography (GC) and liquid chromatography (LC), have been used to determine these proxies. Here we present a sensitive method for determining all three in a single normal phase high performance LC-atmospheric pressure chemical ionization mass spectrometry (NP-HPLC-APCI-MS) analysis. Each of the long chain alkenones and long chain diols was separated and unambiguously identified from the accurate masses and characteristic fragmentation during multiple stage MS analysis (MS2). Comparison of conventional GC and HPLC-MS methods showed similar results for UK'37 and LDI, respectively, using diverse environmental samples and an Emiliania huxleyi culture. Including the three sea surface temperature (SST) proxies; the NP-HPLC-APCI-MS method in fact allows simultaneous determination of nine paleoenvironmental proxies. The extent to which the ecology of the source organisms (ecological bias) influences lipid composition and thereby the reconstructed temperature values was demonstrated by applying the new method to a sediment core from the Sea of Marmara, covering the last 21 kyr BP. Reconstructed SST values differed considerably between the proxies for the Last Glacial Maximum (LGM) and the period of Sapropel S1 formation at ca. 10 kyr BP, whereas the trends during the late Holocene were similar. Changes in the composition of alkenone-producing species at the transition from the LGM to the Bølling/Allerød (B/A) were inferred from unreasonably high UK'37-derived SST values (ca. 20 °C) during the LGM. We ascribe discrepancies between the reconstructed temperature records during S1 deposition to habitat change, e.g. a different depth due to changes in nutrient availability.