ALBERT

Description: The reconstruction of thermal history is essential for evaluating the potential for hydrocarbon generation within sedimentary basins. Magnetic techniques provide an alternative to more traditional methods to study the geothermal history of sedimentary basins (such as illite crystallinity and vitrinite reflectance), which are often associated with significant uncertainty. In this paper the application of various magnetic geothermometers to the western Karoo Basin of South Africa are evaluated. Three magnetic experiments were conducted on samples from stratigraphic borehole G39977 to determine the thermal effect of large scale dolerite intrusions on the sedimentary strata of the Karoo Supergroup in western South Africa. Alteration index (A 40 ) data indicate maximum acquired temperatures for the sedimentary units ranging between 200°C and 650°C, with the highest temperatures restricted to short distances (less than half the sill thicknesses) within the contact aureoles. Both magnetostratigraphy and anisotropy of low field magnetic susceptibility (AMS) data confirm that re-magnetization of magnetic fabric does not exceed distances more than half the sill thicknesses. Our results indicate the general elevation of the palaeotemperatures of the organic-rich sedimentary rocks of the Ecca Group to temperatures where hydrocarbons are normally converted into gas. Importantly, it is clear from this study that the greatest thermal effects of the sill intrusions on the sedimentary strata are limited to the contact aureoles, suggesting that there is an, as yet unquantified, potential for hydrocarbon resources remaining between these intrusions.

Description: 〈span〉〈div〉Abstract〈/div〉The ultramafic-mafic layered igneous Molopo Farms Complex straddles the border between South Africa and Botswana. Younger cover obscures this igneous complex and its country rocks, which are generally assigned to the Paleoproterozoic Transvaal Supergroup. The area intruded by the complex is characterized by abutting and contrasting successions of the upper Transvaal Supergroup (i.e., the Pretoria and Postmasburg groups), the correlation of which is critical to understanding the first significant build-up in atmospheric oxygen, also known as the Great Oxidation Event. Recent dating of the Postmasburg Group necessitates a reinterpretation of Transvaal Supergroup stratigraphy involving a 200 million year downward revision of the Postmasburg Group relative to the Pretoria Group. The geology of the area intruded by the Molopo Farms Complex may provide important insights into this correlation model. Here we report a 〈sup〉207〈/sup〉Pb/〈sup〉206〈/sup〉Pb ID-TIMS baddeleyite date of 2054 ± 5 Ma from a gabbroic sample from the Molopo Farms Complex in South Africa, and an U-Pb zircon date of 2056 ± 10 Ma from a highly altered and metamorphosed quartzite in direct contact with ultramafic rocks of the complex in Botswana. We interpret these as crystallization ages of the Complex, which are within error of the 2056 to 2055 Ma age of the Bushveld Complex. Also reported from drill core intersections are U-Pb LA-ICP-MS detrital zircon age data from quartzite samples of both the floor and roof country rock. The roof rock detrital zircon age populations are comparable to those of the Paleoproterozoic Waterberg Group. Age populations in floor rocks are generally similar to those of the Pretoria Group, with the addition of ~2050 Ma populations, which likely reflect the metamorphic aureole of the complex on distinctly recrystallized country rock. A revised pre-Kalahari regional geology of the Molopo Farms Complex in South Africa, that incorporates the Pretoria Group in the area, implies an unconformable relationship with the Potsmasburg Group. Future recognition of such an unconformable relationship in drill core will ultimately resolve the problem of Transvaal strata correlation.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The ultramafic-mafic layered igneous Molopo Farms Complex straddles the border between South Africa and Botswana. Younger cover obscures this igneous complex and its country rocks, which are generally assigned to the Paleoproterozoic Transvaal Supergroup. The area intruded by the complex is characterized by abutting and contrasting successions of the upper Transvaal Supergroup (i.e., the Pretoria and Postmasburg groups), the correlation of which is critical to understanding the first significant build-up in atmospheric oxygen, also known as the Great Oxidation Event. Recent dating of the Postmasburg Group necessitates a reinterpretation of Transvaal Supergroup stratigraphy involving a 200 million year downward revision of the Postmasburg Group relative to the Pretoria Group. The geology of the area intruded by the Molopo Farms Complex may provide important insights into this correlation model. Here we report a 〈sup〉207〈/sup〉Pb/〈sup〉206〈/sup〉Pb ID-TIMS baddeleyite date of 2054 ± 5 Ma from a gabbroic sample from the Molopo Farms Complex in South Africa, and an U-Pb zircon date of 2056 ± 10 Ma from a highly altered and metamorphosed quartzite in direct contact with ultramafic rocks of the complex in Botswana. We interpret these as crystallization ages of the Complex, which are within error of the 2056 to 2055 Ma age of the Bushveld Complex. Also reported from drill core intersections are U-Pb LA-ICP-MS detrital zircon age data from quartzite samples of both the floor and roof country rock. The roof rock detrital zircon age populations are comparable to those of the Paleoproterozoic Waterberg Group. Age populations in floor rocks are generally similar to those of the Pretoria Group, with the addition of ~2050 Ma populations, which likely reflect the metamorphic aureole of the complex on distinctly recrystallized country rock. A revised pre-Kalahari regional geology of the Molopo Farms Complex in South Africa, that incorporates the Pretoria Group in the area, implies an unconformable relationship with the Potsmasburg Group. Future recognition of such an unconformable relationship in drill core will ultimately resolve the problem of Transvaal strata correlation.〈/span〉

Description: The Karoo Basin of South Africa is of economic importance for its large coal reserves but has in recent years also been in the spotlight due to the possibility of extensive shale gas reserves. Reconstruction of the thermal history of the Karoo Basin is essential for evaluating the potential hydrocarbon generation within this Late Carboniferous – Middle Jurassic sedimentary basin. Magnetic techniques provide an alternative approach in comparison to more traditional methods to study the geothermal history of sedimentary basins (such as illite crystallinity and vitrinite reflectance), which are often associated with significant uncertainty. In this paper variations in the thermal history across the Karoo Basin as a result of heating by the Karoo LIP are evaluated using different magnetic "geothermometers". These include palaeomagnetism (baked contact test), thermomagnetic analysis (alteration index method) and anisotropy of magnetic susceptibility (AMS). Although these techniques were successful in identifying a variation in metamorphic effects adjacent to contact aureoles, only the alternating index (A 40 ) provides a means of estimating peak temperatures. Our results indicate a regional elevation of palaeotemperatures of the organic-rich sedimentary rocks of the Ecca Group to temperatures where hydrocarbons are normally converted into gas. This study shows that the greatest thermal effects of the sill intrusions on the sedimentary strata are limited to the contact aureoles, suggesting that there is an, as yet unquantified, potential for hydrocarbon resources remaining in strata between these intrusions. An increase in the paleotemperatures from 200°C in the southwest to 400°C in the northeast of the basin is observed. We hypothesize that this trend is mainly due to differences in thermal conductivity of the different sedimentary rock types across the basin as the Karoo Basin transgresses from tight low porosity marine shales in the south and southwest towards more lacustrine mudstone and porous sandstone towards the northeast.

Description: Oceans are a net source of molecular hydrogen (H2) to the atmosphere. The production of marine H2 is assumed to be mainly biological by N2 fixation, but photochemical pathways are also discussed. We present measurements of mole fraction and isotopic composition of dissolved and atmospheric H2 from the southern and northern Atlantic between 2008 and 2010. In total almost 400 samples were taken during five cruises along a transect between Punta Arenas (Chile) and Bremerhaven (Germany), as well as at the coast of Mauretania. The isotopic source signatures of dissolved H2 extracted from surface water are highly deuterium-depleted and correlate negatively with temperature, showing δD values of (−629 ± 54) ‰ for water temperatures at (27 ± 3) °C and (−249 ± 88) ‰ below (19 ± 1) °C. The results for warmer water masses are consistent with biological production of H2. This is the first time that marine H2 excess has been directly attributed to biological production by isotope measurements. However, the isotope values obtained in the colder water masses indicate that beside possible biological production a significant different source should be considered. The atmospheric measurements show distinct differences between both hemispheres as well as between seasons. Results from the global chemistry transport model TM5 reproduce the measured H2 mole fractions and isotopic composition well. The climatological global oceanic emissions from the GEMS database are in line with our data and previously published flux calculations. The good agreement between measurements and model results demonstrates that both the magnitude and the isotopic signature of the main components of the marine H2 cycle are in general adequately represented in current atmospheric models despite a proposed source different from biological production or a substantial underestimation of nitrogen fixation by several authors.

Description: Coastal upwelling regions have been identified as sites of enhanced CH4 emissions to the atmosphere. The coastal upwelling area off Mauritania (NW Africa) is one of the most biologically productive regions of the world's ocean but its CH4 emissions have not been quantified so far. More than 1000 measurements of atmospheric and dissolved CH4 in the surface layer in the upwelling area off Mauritania were performed as part of the German SOPRAN (Surface Ocean Processes in the Anthropocene) study during two cruises in March/April 2005 (P320/1) and February 2007 (P348). During P348 enhanced CH4 saturations of up to 200% were found close to the coast and were associated with upwelling of South Atlantic Central Water. An area-weighted, seasonally adjusted estimate yielded overall annual CH4 emissions in the range from 1.6 to 2.9 Gg CH4. Thus the upwelling area off Mauritania represents a regional hot spot of CH4 emissions but seems to be of minor importance for the global oceanic CH4 emissions.

Description: Coastal seas may account for more than 75 % of global oceanic methane emissions. There, methane is mainly produced microbially in anoxic sediments from where it can escape to the overlying water column. Aerobic methane oxidation (MOx) in the water column acts as a biological filter reducing the amount of methane that eventually evades to the atmosphere. The efficiency of the MOx filter is potentially controlled by the availability of dissolved methane and oxygen, as well as temperature, salinity, and hydrographic dynamics, and all of these factors undergo strong temporal fluctuations in coastal ecosystems. In order to elucidate the key environmental controls, specifically the effect of oxygen availability, on MOx in a seasonally stratified and hypoxic coastal marine setting, we conducted a 2-year time-series study with measurements of MOx and physico-chemical water column parameters in a coastal inlet in the southwestern Baltic Sea (Eckernförde Bay). We found that MOx rates always increased toward the seafloor, but were not directly linked to methane concentrations. MOx exhibited a strong seasonal variability, with maximum rates (up to 11.6 nmol l−1 d−1) during summer stratification when oxygen concentrations were lowest and bottom-water temperatures were highest. Under these conditions, 70–95 % of the sediment-released methane was oxidized, whereas only 40–60 % were consumed during the mixed and oxygenated periods. Laboratory experiments with manipulated oxygen concentrations in the range of 0.2–220 µmol l−1 revealed a sub-micromolar oxygen-optimum for MOx at the study site. In contrast, the fraction of methane-carbon incorporation into the bacterial biomass (compared to the total amount of oxidised methane) was up to 38-fold higher at saturated oxygen concentrations, suggesting a different partitioning of catabolic and anabolic processes under oxygen-replete and oxygen-starved conditions, respectively. Our results underscore the importance of MOx in mitigating methane emission from coastal waters and indicate an organism-level adaptation of the water column methanotrophs to hypoxic conditions.

Description: The Eastern Tropical Pacific (ETP) is believed to be one of the largest marine sources of the greenhouse gas nitrous oxide N2O). Future N2Oemissions from the ETP are highly uncertain because oxygen minimum zones are expected to expand, affecting both regional production and consumption of N2O. Here we assess three primary uncertainties in how N2O may respond to changing O2 levels: (1) the relationship between N2O production and O2 (is it linear or exponential at low O2 concentrations?), (2) the cutoff point at which net N2O production switches to net N2O consumption (uncertainties in this parameterization can lead to differences in model ETP N2O concentrations of more than 20%), and (3) the rate of net N2O consumption at low O2. Based on the MEMENTO database, which is the largest N2O dataset currently available, we find that N2O production in the ETP increases linearly rather than exponentially with decreasing O2. Additionally, net N2O consumption switches to net N2O production at ~ 10 μM O2, a value in line with recent studies that suggest consumption occurs on a larger scale than previously thought. N2O consumption is on the order of 0.129 mmol N2O m−3 yr−1 in the Peru–Chile Undercurrent. Based on these findings, it appears that recent studies substantially overestimated N2O production in the ETP. In light of expected deoxygenation, future N2O production is still uncertain, but due to higher-than-expected consumption levels, it is possible that N2Oconcentrations may decrease rather than increase as oxygen minimum zones expand.

Description: Here we present results of the first comprehensive study of sulphur compounds and methane in the oligotrophic tropical West Pacific Ocean. The concentrations of dimethylsuphide (DMS), dimethylsulphoniopropionate (DMSP), dimethylsulphoxide (DMSO), and methane (CH4), as well as various phytoplankton marker pigments in the surface ocean were measured along a north-south transit from Japan to Australia in October 2009. DMS (0.9 nmol l−1), dissolved DMSP (DMSPd, 1.6 nmol l−1) and particulate DMSP (DMSPp, 2 nmol l−1) concentrations were generally low, while dissolved DMSO (DMSOd, 4.4 nmol l−1) and particulate DMSO (DMSOp, 11.5 nmol l−1) concentrations were comparably enhanced. Positive correlations were found between DMSO and DMSP as well as DMSP and DMSO with chlorophyll a, which suggests a similar source for both compounds. Similar phytoplankton groups were identified as being important for the DMSO and DMSP pool, thus, the same algae taxa might produce both DMSP and DMSO. In contrast, phytoplankton seemed to play only a minor role for the DMS distribution in the western Pacific Ocean. The observed DMSPp : DMSOp ratios were very low and seem to be characteristic of oligotrophic tropical waters representing the extreme endpoint of the global DMSPp : DMSOp ratio vs. SST relationship. It is most likely that nutrient limitation and oxidative stress in the tropical West Pacific Ocean triggered enhanced DMSO production leading to an accumulation of DMSO in the sea surface. Positive correlations between DMSPd and CH4, as well as between DMSO (particulate and total) and CH4, were found along the transit. We conclude that both DMSP and DMSO serve as substrates for methanogenic bacteria in the western Pacific Ocean.

Description: The Arabian Sea harbours one of the three major oxygen minimum zones (OMZs) in the world's oceans, and it alone is estimated to account for ~10–20 % of global oceanic nitrogen (N) loss. While actual rate measurements have been few, the consistently high accumulation of nitrite (NO2−) coinciding with suboxic conditions in the central-northeastern part of the Arabian Sea has led to the general belief that this is the region where active N-loss takes place. Most subsequent field studies on N-loss have thus been drawn almost exclusively to the central-NE. However, a recent study measured only low to undetectable N-loss activities in this region, compared to orders of magnitude higher rates measured towards the Omani Shelf where little NO2− accumulated (Jensen et al., 2011). In this paper, we further explore this discrepancy by comparing the NO2−-producing and consuming processes, and examining the relationship between the overall NO2− balance and active N-loss in the Arabian Sea. Based on a combination of 15N-incubation experiments, functional gene expression analyses, nutrient profiling and flux modeling, our results showed that NO2− accumulated in the central-NE Arabian Sea due to a net production via primarily active nitrate (NO3−) reduction and to a certain extent ammonia oxidation. Meanwhile, NO2− consumption via anammox, denitrification and dissimilatory nitrate/nitrite reduction to ammonium (NH4+) were hardly detectable in this region, though some loss to NO2− oxidation was predicted from modeled NO3− changes. No significant correlation was found between NO2− and N-loss rates (p〉0.05). This discrepancy between NO2− accumulation and lack of active N-loss in the central-NE Arabian Sea is best explained by the deficiency of labile organic matter that is directly needed for further NO2− reduction to N2O, N2 and NH4+, and indirectly for the remineralized NH4+ required by anammox. Altogether, our data do not support the long-held view that NO2− accumulation is a direct activity indicator of N-loss in the Arabian Sea or other OMZs. Instead, NO2− accumulation more likely corresponds to long-term integrated N-loss that has passed the prime of high and/or consistent in situ activities.