Auslöser für größtes Massenaussterben der Erdgeschichte identifiziert Neue Studie liefert umfassende Rekonstruktion der Perm-Trias-Grenze 19.10.2020/Kiel. Vor 252 Millionen Jahren starben am Übergang vom Erdzeitalter des Perm zu dem der Trias die meisten damals auf der Erde existierenden Lebensformen aus. Mit Hilfe neuester Analysemethoden und detaillierter Modellrechnungen ist es Wissenschaftlerinnen und Wissenschaftlern des GEOMAR Helmholtz-Zentrums für Ozeanforschung Kiel in Kooperation mit dem Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum und internationalen Partnern jetzt erstmals gelungen, die geochemischen Abläufe, die zu diesem Massenaussterben geführt haben, schlüssig nachzuvollziehen. Die Studie ist heute in der Fachzeitschrift Nature Geoscience erschienen.

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The loss of carbonate production during the Toarcian Oceanic Anoxic Event (T-OAE, ca. 183 Ma) is hypothesized to have been at least partly triggered by ocean acidification linked to magmatism from the Karoo-Ferrar large igneous province (southern Africa and Antarctica). However, the dynamics of acidification have never been directly quantified across the T-OAE. Here, we present the first record of temporal evolution of seawater pH spanning the late Pliensbachian and early Toarcian from the Lusitanian Basin (Portugal) reconstructed on the basis of boron isotopic composition (δ11B) of brachiopod shells. δ11B declines by ~1‰ across the Pliensbachian-Toarcian boundary (Pl-To) and attains the lowest values (~12.5‰) just prior to and within the T-OAE, followed by fluctuations and a moderately increasing trend afterwards. The decline in δ11B coincides with decreasing bulk CaCO3 content, in parallel with the two-phase decline in carbonate production observed at global scales and with changes in pCO2 derived from stomatal indices. Seawater pH had declined significantly already prior to the T-OAE, probably due to the repeated emissions of volcanogenic CO2. During the earliest phase of the T-OAE, pH increased for a short period, likely due to intensified continental weathering and organic carbon burial, resulting in atmospheric CO2 drawdown. Subsequently, pH dropped again, reaching the minimum in the middle of the T-OAE. The early Toarcian marine extinction and carbonate collapse were thus driven, in part, by ocean acidification, similar to other Phanerozoic events caused by major CO2 emissions and warming.

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Coral-based reconstructions of sea surface temperatures (SSTs) using Sr/Ca, U/Ca and δ 18 O are important tools for quantitative analysis of past climate variabilities. However, post-depositional alteration of coral aragonite, particularly early diagenesis, restrict the accuracy of calibrated proxies even on young corals. Considering the diagenetic effects, we present new Mid to Late Holocene SST reconstructions on well-dated (U/Th: ∼70 yr to 5.4 ka) fossil Porites sp. collected from the Society Islands, French Polynesia. For few corals, quality pre-screening routines revealed the presence of secondary aragonite needles inside primary pore space, resulting in a mean increase in Sr/Ca ratios between 5-30%, in contrast to the massive skeletal parts. Characterized by a Sr/Ca above 10 mmol/mol, we interpret this value as the threshold between diagenetically altered and unaltered coral material. At a high-resolution, observed intra-skeletal variability of 5.4 to 9.9 mmol/mol probably reflects the physiological control of corals over their trace metal uptake, and individual variations controlled by CaCO 3 − precipitation rates. Overall, the Sr/Ca, U/Ca and δ 18 O trends are well correlated, but we observed a significant offset up to ± 7 • C among the proxies on derived palaeo-SST estimates. It appears that the related alteration process tends to amplify temperature extremes, resulting in increased SST-U/Ca and SST-Sr/Ca gradients, and consequently their apparent temperature sensitivities. A relative SST reconstruction is still feasible by normalizing our records to their individual mean value defined as SST. This approach shows that SST records derived from different proxies agree with an amplitudinal variability of up to ± 2 • C with respect to their Holocene mean value. Higher SST values than the mean SSTs (Holocene warm periods) were recorded from ∼1.8 to ∼2.8 ka (Interval I), ∼3.7 to 4.0 ka (Interval III) and before ∼5 ka, while lower SST values (Holocene cold periods, Interval II and IV) were recorded in between. The ensuing SST periodicity of ∼1.5 ka in the Society Islands record is in line with the solar activity reconstructed from 10 Be and 14 C production (Vonmoos et al., 2006), emphasizing the role of solar activity on climate variability during the Late Holocene.

Abstract. In the last few decades and in the near future CO2-induced ocean acidification is potentially a big threat to marine calcite-shelled animals (e.g. brachiopods, bivalves, corals and gastropods). Despite the great number of studies focusing on the effects of acidification on shell growth, metabolism, shell dissolution and shell repair, the consequences for biomineral formation remain poorly understood. Only a few studies have addressed the impact of ocean acidification on shell microstructure and geochemistry. In this study, a detailed microstructure and stable isotope geochemistry investigation was performed on nine adult brachiopod specimens of Magellania venosa (Dixon, 1789). These were grown in the natural environment as well as in controlled culturing experiments under different pH conditions (ranging from 7.35 to 8.15±0.05) over different time intervals (214 to 335 days). Details of shell microstructural features, such as thickness of the primary layer, density and size of endopunctae and morphology of the basic structural unit of the secondary layer were analysed using scanning electron microscopy. Stable isotope compositions (δ13C and δ18O) were tested from the secondary shell layer along shell ontogenetic increments in both dorsal and ventral valves. Based on our comprehensive dataset, we observed that, under low-pH conditions, M. venosa produced a more organic-rich shell with higher density of and larger endopunctae, and smaller secondary layer fibres. Also, increasingly negative δ13C and δ18O values are recorded by the shell produced during culturing and are related to the CO2 source in the culture set-up. Both the microstructural changes and the stable isotope results are similar to observations on brachiopods from the fossil record and strongly support the value of brachiopods as robust archives of proxies for studying ocean acidification events in the geologic past.

Lakes and reservoirs play an important role in the carbon cycle, and therefore monitoring their metabolic rates is essential. The triple oxygen-isotope anomaly of dissolved O 2 [ 17 = ln(1+δ 17 O) − 0.518 × ln(1+δ 18 O)] offers a new, in situ, perspective on primary production, yet little is known about 17 from freshwater systems. We investigated the 17 together with the oxygen : argon ratio [ (O 2 / Ar)] in the subtropical Feitsui Reservoir in Taiwan from June 2014 to July 2015. Here, we present the seasonal variations in 17 , GP (gross production), NP (net production) and the NP / GP (net to gross ratio) in association with environmental parameters. The 17 varied with depth and season, with values ranging between 26 and 205 per meg. The GP rates were observed to be higher (702 ± 107 mg C m −2 d −1 ) in winter than those (303 ± 66 mg C m −2 d −1 ) recorded during the summer. The overall averaged GP was 220 g C m −2 yr −1 and NP was −3 g C m −2 yr −1 , implying the reservoir was net heterotrophic on an annual basis. This is due to negative NP rates from October to February (−198 ± 78 mg C m −2 d −1 ). Comparisons between GP rates obtained from the isotope mass balance approach and 14 C bottle incubation method ( 14 C-GP) showed consistent values on the same order of magnitude with a GP / 14 C-GP ratio of 1.2 ± 1.1. Finally we noted that, although typhoon occurrences were scarce, higher than average 17 values and GP rates were recorded after typhoon events.Published by Copernicus Publications on behalf of the European Geosciences Union.