ALBERT

Beschreibung: This study summarizes organic carbon isotope (δ13C) and total organic carbon (TOC) data from a series of tests undertaken to provide an appropriate methodology for pre-analysis treatment of mudstones from an Upper Carboniferous sedimentary succession, in order to develop a consistent preparation procedure. The main treatments involved removing both inorganic carbonate and hydrocarbons (which might be extraneous) before δ13C and TOC analysis. The results show that decarbonating using hydrochloric acid causes significant reduction in δ13C and total carbon (TC) of the bulk material due to the removal of inorganic carbonate. These changes are most pronounced where soluble calcium carbonate (rather than Ca-Mg-Fe carbonate) is present. Deoiled samples show only slightly higher mean δ13C where visible bitumen was extracted from the bulk sample. Moreover, the isotopic signatures of the extracts are closely correlated to those of their respective bulk samples, suggesting that small yields of hydrocarbons were generated in situ with no isotopic fractionation. In addition, further δ13C and TC analyses were performed on samples where mixing of oil-based drilling mud with brecciated core material had been undertaken. Brecciated mudstone material did not display distinct isotopic signals compared to the surrounding fine-grained material. Overall we show that the most accurate assessment of bulk organic carbon isotopes and concentration in these samples can be achieved through decarbonating the material prior to measurement via the ‘rinse method’. However, our results support recent findings that pre-analysis acid treatments can cause variable and unpredictable errors in δ13C and TOC values. We believe that, despite these uncertainties, the findings presented here can be applied to paleoenvironmental studies on organic matter contained within sedimentary rocks over a range of geological ages and compositions.

Beschreibung: Mixed-layer depth (MLD) is often used in a mixed-layer heat budget to relate air-sea exchange to changes in the near-surface ocean temperature. In this study, reanalysis heat flux products and profiles from a 15 year time series of high-resolution, near-repeat expendable bathythermograph/expendable conductivity-temperature-depth (XBT/XCTD) sampling in Drake Passage are used to examine the nature of MLD variations and their impact on a first-order, one-dimensional heat budget for the upper ocean in the regions north and south of the Polar Front. Results show that temperature and density criteria yield different MLD estimates, and that these estimates can be sensitive to the choice of threshold. The difficulty of defining MLD in low-stratification regions, the large amplitude of wintertime MLD (up to 700 m in Drake Passage), and the natural small-scale variability of the upper ocean result in considerable cast-to-cast variability in MLD, with changes of up to 200 m over 10 km horizontal distance. In contrast, the heat content over a fixed-depth interval of the upper ocean shows greater cast-to-cast stability and clearly measures the ocean response to surface heat fluxes. In particular, an annual cycle in upper ocean heat content is in good agreement with the annual cycle in heat flux forcing, which explains ∼24% of the variance in heat content above 400 m depth north of the Polar Front and ∼63% of the variance in heat content south of the Polar Front.

Beschreibung: Observations made near the Celtic Sea shelf edge are used to investigate the interaction between wind generated near-inertial oscillations and the semi-diurnal internal tide. Linear, baroclinic energy fluxes within the near-inertial ( f ) and semi-diurnal (M 2 ) wave bands are calculated from measurements of velocity and density structure at two moorings located 40 km from the internal tidal generation zone. Over the two week deployment period the semi-diurnal tide drove 28-48 W m -1 of energy directly on-shelf. Little spring-neap variability could be detected. Horizontal near-inertial energy fluxes were an order of magnitude weaker, but non-linear interaction between the vertical shear of inertial oscillations and the vertical velocity associated with the semi-diurnal internal tide led to a 25-43% increase in positive on-shelf energy flux. The phase relationship between f and M 2 determines whether this non-linear interaction enhances or dampens the linear tidal component of the flux, and introduces a 2-day counter clockwise beating to the energy transport. Two very clear contrasting regimes of (a) tidally and (b) inertially driven shear and energy flux are captured in the observations.

Beschreibung: Geodynamic models of mantle convection predict that Mexico and western North America share a history of dynamic support. We calculate admittance between gravity and topography, which indicates that the elastic thickness of the plate in Mexico is 11 km and in western North America it is 12 km. Admittance at wavelengths 〉 500 km in these regions suggests that topography is partly supported by sub-crustal processes. These results corroborate estimates of residual topography from isostatic calculations and suggest that the amount of North American topography supported by the mantle may exceed 1 km. The Cenozoic history of magmatism, sedimentary flux, thermochronometric denudation estimates and uplifted marine terraces imply that North American lithosphere was uplifted and eroded during the last 30 Ma. We jointly invert 533 Mexican and North American longitudinal river profiles to reconstruct a continent-scale rock uplift rate history. Uplift rate is permitted to vary in space and time. Erosional parameters are calibrated using incision rate data in southwest Mexico and the Colorado Plateau. Calculated rock uplift rates were 0.15–0.2 mm/yr between 25–10 Ma. Central Mexico experienced the highest uplift rates. Central and southern Mexico continued to uplift at 0.1 mm/yr until recent times. This uplift history is corroborated by independent constraints. We predict clastic flux to the Gulf of Mexico and compare it to independent estimates. We tentatively suggest that the loop between uplift, erosion and deposition can be closed here. Mexico's staged uplift history suggests that its dynamic support has changed during the last 30 Ma. This article is protected by copyright. All rights reserved.

Beschreibung: A 16-year record of expendable bathythermograph (XBT) transects across Drake Passage is used to examine variability in upper ocean heat content that is not associated with the annual cycle. Links between upper ocean heat content and anomalous heat fluxes, winds, two large-scale climate indices, and mesoscale eddies and meanders are examined. Results suggest that interannual variations in surface heat fluxes explain ~5-10% of the variance in upper-ocean heat content. Anomalous surface heat fluxes are linked to meridional wind anomalies upstream of Drake Passage, which in turn are linked to forcing by El Niño / Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). ENSO and SAM are correlated with upper ocean heat content at near-zero lags, and statistically significant correlations occur at longer time lags as well. The impact of mesoscale eddies and meanders on upper ocean heat content is explored using a tracked eddy database. An empirical relationship is constructed relating upper ocean heat content anomalies to eddy length scales and amplitudes. Eddies and meanders are estimated to account for more than one third of the non-annual-cycle variance in Drake Passage upper-ocean heat content.