Geochemistry, lithology and maceral analysis at DSDP Hole 71-511

The quantity, type, and maturity of the organic matter in Recent through Upper Jurassic sediments from the Falkland Plateau, DSDP Site 511, have been determined. Sediments were investigated for their hydrocarbon potential by organic carbon and Rock-Eval pyrolysis. Kerogen concentrates were prepared and analyzed in reflected and transmitted light to determine vitrinite reflectance and maceral content. Total extractable organic compounds were analyzed for their elemental composition, and the fraction of the nonaromatic hydrocarbons was determined by capillary column gas chromatography and combined gas chromatography/mass spectrometry. Three main classes of organic matter can be determined at DSDP Site 511 by a qualitative and quantitative evaluation of microscopic and geochemical results. The Upper Jurassic to lower Albian black shales contain high amounts of organic matter of dominantly marine origin. The content of terrigenous organic matter increases at the base of the black shales, whereas the shallowest black shales near the Aptian/Albian boundary are transitional in composition, with increasing amounts of inert, partly oxidized organic matter which is the dominant component in all Albian through Tertiary sediments investigated. The organic matter in the black shales has a low level of maturity and has not yet reached the onset of thermal hydrocarbon generation. This is demonstrated by the low amounts of total extractable organic compounds, low percentages of hydrocarbons, and the pattern and composition of nonaromatic hydrocarbons. The observed reflectance of huminite and vitrinite particles (between 0.4% and 0.5% Ro at bottom-hole depth of 632 m) is consistent with this interpretation. Several geochemical parameters indicate, however, a rapid increase in the maturation of organic matter with depth of burial. This appears to result from the relatively high heat flow observed at Site 511. If we relate the level of maturation of the black shales at the bottom of Hole 511 to their present shallow depth of burial, they appear rather mature. On the basis of comparisons with other sedimentary basins of a known geothermal history, a somewhat higher paleotemperature gradient and/or additional overburden are required to give the observed maturity at shallow depth. A comparison with contemporaneous sediments of DSDP Site 361, Cape Basin, which was the basin adjacent and to the north of the Falkland Plateau during the early stages of the South Atlantic Ocean, demonstrates differences in sedimentological features and in the nature of sedimentary organic matter. We interpret these differences to be the result of the different geological settings for Sites 361 and 511.

Sea-suface microlayer (SML) and underlying water (ULW) samples focusing on CDOM spectral characteristics during METEOR cruise M91

The coastal upwelling system off the coast of Peru is characterized by high biological activity and a pronounced subsurface oxygen minimum zone, as well as associated emissions of atmospheric trace gases such as N2O, CH4 and CO2. From 3 to 23 December 2012, R/V Meteor (M91) cruise took place in the Peruvian upwelling system between 4.59 and 15.4°S, and 82.0 to 77.5°W. During M91 we investigated the composition of the sea-surface microlayer (SML), the oceanic uppermost boundary directly subject to high solar radiation, often enriched in specific organic compounds of biological origin like chromophoric dissolved organic matter (CDOM) and marine gels. In the SML, the continuous photochemical and microbial recycling of organic matter may strongly influence gas exchange between marine systems and the atmosphere. We analyzed SML and underlying water (ULW) samples at 38 stations focusing on CDOM spectral characteristics as indicator of photochemical and microbial alteration processes. CDOM composition was characterized by spectral slope (S) values and excitation-emission matrix fluorescence (EEMs), which allow us to track changes in molecular weight (MW) of DOM, and to determine potential DOM sources and sinks. Spectral slope S varied between 0.012 to 0.043 1 nm-1 and was quite similar between SML and ULW, with no significant differences between the two compartments. Higher S values were observed in the ULW of the southern stations below 15°S. By EEMs, we identified five fluorescent components (F1-5) of the CDOM pool, of which two had excitation/emission characteristics of amino-acid-like fluorophores (F1, F4) and were highly enriched in the SML, with a median ratio SML : ULW of 1.5 for both fluorophores. In the study region, values for CDOM absorption ranged from 0.07 to 1.47 m-1. CDOM was generally highly concentrated in the SML, with a median enrichment with respect to the ULW of 1.2. CDOM composition and changes in spectral slope properties suggested a local microbial release of DOM directly in the SML as a response to light exposure in this extreme environment. In a conceptual model of the sources and modifications of optically active DOM in the SML and underlying seawater (ULW), we describe processes we think may take place (Fig. 1); the production of CDOM of higher MW by microbial release through growth, exudation and lysis in the euphotic zone, includes the identified fluorophores (F1, F2, F3, F4, F5). Specific amino-acid-like fluorophores (F1, F4) accumulate in the SML with respect to the ULW, as photochemistry may enhance microbial CDOM release by (a) photoprotection mechanisms and (b) cell-lysis processes. Microbial and photochemical degradation are potential sinks of the amino-acid-like fluorophores (F1, F4), and potential sources of reworked and more refractory humic-like components (F2, F3, F5). In the highly productive upwelling region along the Peruvian coast, the interplay of microbial and photochemical processes controls the enrichment of amino-acid-like CDOM in the SML. We discuss potential implications for air-sea gas exchange in this area.

Chemical characterization of marine sub-micrometer aerosol particles during the MERIAN cruise MSM18/3 from the Cape Verde island Sao Vicente to Gabun (Atlantic Ocean)

Sub-micron marine aerosol particles (PM1) were collected during the MERIAN cruise MSM 18/3 between 22 June 2011 and 21 July 2011 from the Cape Verde island Sao Vicente to Gabun crossing the tropical Atlantic Ocean and passing equatorial upwelling areas. According to air mass origin and chemical composition of the aerosol particles, three main regimes could be established. Aerosol particles in the first part of the cruise were mainly of marine origin, in the second part was marine and slightly biomass burning influenced (increasing tendency) and in the in last part of the cruise, approaching the African mainland, biomass burning influences became dominant. Generally aerosols were dominated by sulfate (caverage = 1.99 µg/m**3) and ammonium ions (caverage = 0.72 µg/m**3) that are well correlated and slightly increasing along the cruise. High concentrations of water insoluble organic carbon (WISOC) averaging 0.51 µg/m**3 were found probably attributed to the high oceanic productivity in this region. Water soluble organic carbon (WSOC) was strongly increasing along the cruise from concentrations of 0.26 µg/m**3 in the mainly marine influenced part to concentrations up to 3.3 µg/m**3 that are probably caused by biomass burning influences. Major organic constituents were oxalic acid, methansulfonic acid (MSA) and aliphatic amines. MSA concentrations were quite constant along the cruise (caverage = 43 ng/m**3). While aliphatic amines were more abundant in the first mainly marine influenced part with concentrations of about 20 ng/m**3, oxalic acid showed the opposite pattern with average concentrations of 12 ng/m**3 in the marine and 158 ng/m**3 in the biomass burning influenced part. The alpha dicarbonyl compounds glyoxal and methylglyoxal were detected in the aerosol particles in the low ng/m**3 range and followed oxalic acid closely. MSA and aliphatic amines accounted for biogenic marine (secondary) aerosol constituents whereas oxalic acid and the alpha dicarbonyl compounds were believed to result mainly from biomass burning. N-alkane concentrations increased along the cruise from 0.81 to 4.66 ng/m**3, PAHs and hopanes were abundant in the last part of the cruise (caverage of PAHs = 0.13 ng/m**3, caverage of hopanes = 0.19 ng/m**3). Levoglucosan was identified in several samples of the last part of the cruise in concentrations around 2 ng/m**3, pointing to (aged) biomass burning influences. The investigated organic compounds could explain 9.5% of WSOC in the mainly marine influenced part (dominating compounds: aliphatic amines and MSA) and 2.7% of WSOC in the biomass burning influenced part (dominating compound: oxalic acid) of the cruise.

Biogeochemical parameters (oxygen, nitrate, phosphate, TOC and strontium) at three sites from the Clarion-Clipperton Fracture Zone

The Clarion-Clipperton Fracture Zone (CCFZ) in the Pacific Ocean is characterized by organic carbon-starved sediments and meter-scale oxygen penetration into the sediment. Furthermore, numerous seamounts occur throughout its deep-sea plain, which may serve as conduits for low-temperature hydrothermal circulation of seawater through the oceanic crust. Recent studies in deep-sea environments of the Pacific and Atlantic Oceans have suggested and presented evidence of an exchange of dissolved constituents between the seawater flowing in the basaltic crust and the pore water of the overlying sediments. Through high-resolution pore-water oxygen and nutrient measurements, we examined fluxes and geochemical interactions between the seamount basaltic basement and pore waters of the overlying sediments at three sites located on a radial transect from the foot of Teddy Bare, a small seamount in the CCFZ. At three sites, located 1000, 700 and 400 m away from the foot of the seamount, we found that oxygen concentrations initially decrease with sediment depth but start to increase at depths of 3 and 7 m towards the basaltic basement. NO32- concentrations mirror the oxygen concentration profiles, as they increase with sediment depth but decrease towards the basement. We performed transport reaction modeling and determined at one site the 87Sr/86Sr ratio of the pore water and the bottom water overlying the sediments, which indicated that the 87Sr/86Sr ratio of the pore water at the bottom of the sediment column is similar to the seawater Transport-reaction modeling revealed that (1) the diffusive flux of oxygen from the basaltic basement outpaces the oxygen consumption through organic matter oxidation and nitrification in the basal sediments and (2) the nutrient exchange between the sediment and the underlying basaltic crust occurs at orders-of-magnitude lower rates than between the upper sediment and the overlying bottom water. Our results suggest an upward diffusion of oxygen from seawater circulating within the seamount crust into the overlying basal sediments. The oxygen profiles presented here represent the first of their kind ever measured in the Pacific Ocean, as they indicate an upward flux of molecular oxygen from a basaltic aquifer, something that has so far only been documented - at one other location worldwide - the North Pond site in the Atlantic Ocean. We show that the diffusion of oxygen from the seamount basaltic basement into the overlying pore waters affects the preservation of organic compounds and helps to maintain a completely oxygenated sedimentary column at all 3 sites near the seamount.