Geochemical analysis of nine sections from the Nama Group, Namibia

The first appearance of skeletal metazoans in the late Ediacaran (~550 million years ago; Ma) has been linked to the widespread development of oxygenated oceanic conditions, but a precise spatial and temporal reconstruction of their evolution has not been resolved. Here we consider the evolution of ocean chemistry from ~550 to ~541 Ma across shelf-to-basin transects in the Zaris and Witputs Sub-Basins of the Nama Group, Namibia. New carbon isotope data capture the final stages of the Shuram/Wonoka deep negative C-isotope excursion, and these are complemented with a reconstruction of water column redox dynamics utilising Fe-S-C systematics and the distribution of skeletal and soft-bodied metazoans. Combined, these inter-basinal datasets provide insight into the potential role of ocean redox chemistry during this pivotal interval of major biological innovation. The strongly negative d13C values in the lower parts of the sections reflect both a secular, global change in the C-isotopic composition of Ediacaran seawater, as well as the influence of 'local' basinal effects as shown by the most negative d13C values occurring in the transition from distal to proximal ramp settings. Critical, though, is that the transition to positive d13C values postdates the appearance of calcified metazoans, indicating that the onset of biomineralization did not occur under post-excursion conditions. Significantly, we find that anoxic and ferruginous deeper water column conditions were prevalent during and after the transition to positive d13C that marks the end of the Shuram/Wonoka excursion. Thus, if the C isotope trend reflects the transition to global-scale oxygenation in the aftermath of the oxidation of a large-scale, isotopically light organic carbon pool, it was not sufficient to fully oxygenate the deep ocean. Both sub-basins reveal highly dynamic redox structures, where shallow, inner ramp settings experienced transient oxygenation. Anoxic conditions were caused either by episodic upwelling of deeper anoxic waters or higher rates of productivity. These settings supported short-lived and monospecific skeletal metazoan communities. By contrast, microbial (thrombolite) reefs, found in deeper inner- and mid-ramp settings, supported more biodiverse communities with complex ecologies and large skeletal metazoans. These long-lived reef communities, as well as Ediacaran soft-bodied biotas, are found particularly within transgressive systems, where oxygenation was persistent. We suggest that a mid-ramp position enabled physical ventilation mechanisms for shallow water column oxygenation to operate during flooding and transgressive sea-level rise. Our data support a prominent role for oxygen, and for stable oxygenated conditions in particular, in controlling both the distribution and ecology of Ediacaran skeletal metazoan communities.

Osmium, and stable carbon and oxygen isotope record accross the PETM

In the latest Paleocene an abrupt shift to more negative d13C values has been documented at numerous marine and terrestrial sites (Bralower et al., 1997, doi:10.1130/0091-7613(1997)025<0963:HRROTL>2.3.CO;2; Cramer et al., 1999; Kaiho et al., 1996, doi:10.1029/96PA01021; Kennett and Stott, 1991, doi:10.1038/353225a0; Koch et al., 1992, doi:10.1038/358319a0; Stott et al., 1996; Thomas and Shackleton, 1996, doi:10.1144/GSL.SP.1996.101.01.20; Zachos et al., 1993). This carbon isotope event (CIE) is coincident with oxygen isotope data that indicate warming of surface waters at high latitudes of nearly 4°-6°C (Kennett and Stott, 1991, doi:10.1038/353225a0) and more moderate warming in the subtropics (Thomas et al., 1999, doi:10.1029/1999PA900031). Here we report 187Os/188Os isotope records from the North Atlantic and Indian Oceans which demonstrate a >10% increase in the 187Os/188Os ratio of seawater coincident with the late Paleocene CIE. This excursion to higher 187Os/188Os ratios is consistent with a global increase in weathering rates. The inference of increased chemical weathering during this interval of unusual warmth is significant because it provides empirical evidence supporting the operation of a feedback between chemical weathering rates and warm global climate, which acts to stabilize Earth's climate (Walker et al., 1981). Estimates of the duration of late Paleocene CIE (Bains et al., 1999, doi:10.1126/science.285.5428.724; Bralower et al., 1997, doi:10.1130/0091-7613(1997)025<0963:HRROTL>2.3.CO;2; Norris and Röhl, 1999, doi:10.1038/44545; Röhl et al., 2000, doi:10.1130/0091-7613(2000)28<927:NCFTLP>2.0.CO;2) in conjunction with the Os isotope data imply that intensified chemical weathering in response to warm, humid climates can occur on timescales of 104-105 years. This interpretation requires that the late Paleocene thermal maximum Os isotope excursion be produced mainly by increased Os flux to the ocean rather than a transient excursion to higher 187Os/188Os ratios in river runoff. Although we argue that the former is more likely than the latter, we cannot rule out significant changes in the 187Os/188Os ratio of rivers.

Stable isotope composition of pore waters from ODP Site 195-1200 serpentinite mud, South Chamorrow Seamount

The South Chamorro Seamount is a serpentinite mud volcano near the southern end of the Mariana forearc. The mud volcano was sampled by drilling during Ocean Drilling Program Leg 195. Samples of pore water squeezed from serpentinite mud were analyzed for stable isotope compositions of carbon in dissolved inorganic carbon and methane, sulfur in sulfate and sulfide, and oxygen in sulfate.

Mineralogy and stable isotopic composition of carbonates and sulfide minerals from ODP Leg 164 sites

During Ocean Drilling Program Leg 164, gas hydrates were recovered in the Blake Ridge where the top of the gas hydrate zone lies at about 200 meters below seafloor (mbsf) and the bottom-simulating reflector (BSR) is located at about 450 mbsf. There is no sedimentological discontinuity crossing the BSR. The BSR is disrupted by the salt piercement of the Cape Fear Diapir. The authigenic carbonates (dolomite and siderite) are always present in small amounts (a few weight percent) in the sediments; they are also concentrated in millimeter- to centimeter-sized nodules and layers composed of dolomite above the top of the gas hydrate reservoir, and of siderite below the BSR. In the Blake Ridge, the dolomite/siderite boundary is located near 140 mbsf. The distribution with depth of the d18O values of dolomite and siderite shows a sharp decrease from high values (maximum 7.5 per mil) in the topmost 50 m, to very low values (minimum -2.7 per mil) at 140 mbsf, and at greater depth increase to positive values within the range of 1.8 per mil to 5.0 per mil. The d13C distribution is marked by the rapid increase with greater depth from low values (-31.3 per mil to -11.4 per mil) near 50 mbsf to positive values at 110 mbsf, which remain in the range of 1.7 to 5.4 down to 700 mbsf. Diagenetic carbonates were precipitated in pore waters in which d18O and d13C values were highly modified by strong fractionation effects, both in the water and in the CO2-CH4 systems associated with the formation and dissociation of gas hydrates.

Carbonate fraction, magnesium and strontium concentration, oxygen and carbon isotopes at DSDP Hole 92-600C

Strontium, magnesium, oxygen, and carbon isotope profiles of the carbonate fraction of Hole 600C sediments support the lithologic and petrographic observations of extensive CaCO3 dissolution and recrystallization in the Pliocene basal section. Convective fluid flow through the sediments during the first 1 to 1.5 m.y. of the sedimentary history of these sediments may explain these observations.