High-resolution stable isotope variations in a coral Porites lutea from the Wenno Island, Micronesia: Implications of low-latitude ocean environments

Abstract. Three shallow firn cores were retrieved in the austral summers of 2011/12 and 2013/14 on the ice rises Kupol Ciolkovskogo (KC), Kupol Moskovskij (KM), and Blåskimen Island (BI), all part of Fimbul Ice Shelf (FIS) in western Dronning Maud Land (DML), Antarctica. The cores were dated back to 1958 (KC), 1995 (KM), and 1996 (BI) by annual layer counting using high-resolution oxygen isotope (δ18O) data, and by identifying volcanic horizons using non-sea-salt sulfate (nssSO42−) data. The water stable isotope records show that the atmospheric signature of the annual snow accumulation cycle is well preserved in the firn column, especially at KM and BI. We are able to determine the annual surface mass balance (SMB), as well as the mean SMB values between identified volcanic horizons. Average SMB at the KM and BI sites (0.68 and 0.70 mw. e. yr−1) was higher than at the KC site (0.24 mw. e. yr−1), and there was greater temporal variability as well. Trends in the SMB and δ18O records from the KC core over the period of 1958–2012 agree well with other previously investigated cores in the area, thus the KC site could be considered the most representative of the climate of the region. Cores from KM and BI appear to be more affected by local meteorological conditions and surface topography. Our results suggest that the ice rises are suitable sites for the retrieval of longer firn and ice cores, but that BI has the best preserved seasonal cycles of the three records and is thus the most optimal site for high-resolution studies of temporal variability of the climate signal. Deuterium excess data suggest a possible effect of seasonal moisture transport changes on the annual isotopic signal. In agreement with previous studies, large-scale atmospheric circulation patterns most likely provide the dominant influence on water stable isotope ratios preserved at the core sites.

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Onset of Oceanic Anoxic Event 2 in the Lower Saxony Basin – Insights fromhigh-resolution stable isotope stratigraphy and geochemical modelling

10.5194/egusphere-egu21-8748 ◽

2021 ◽

Author(s):

Pia Müller ◽

Ulrich Heimhofer ◽

Christian Ostertag-Henning

Keyword(s):

Organic Matter ◽

High Resolution ◽

Stable Isotope ◽

Carbon Cycle ◽

Geochemical Modelling ◽

Positive Anomaly ◽

Lower Saxony ◽

Data Set ◽

Oceanic Anoxic Event ◽

Anoxic Event

The Oceanic Anoxic Event (OAE) 2 spanning the Cenomanian-Turonian boundary (93.5 Ma) represents a major perturbation of the global carbon cycle and is marked by organic-rich sediments deposited under oxygen-depleted conditions. In many studies the eruption of the Caribbean LIP is considered to be the cause for rapidly increasing CO2 concentrations and resulting global warming accompanied by widespread oceanic anoxia. In the Lower Saxony Basin of northern Germany, the deposits of the OAE 2 are exposed in several industry drill cores. In this study, the lower part of the OAE 2 has been studied in the HOLCIM 2011-3 drill core. Sedimentary rocks are composed of limestones, marly limestones, marls and black shales and have been analysed with a high-resolution stable isotope approach (approximately one sample every 2 cm) combined with geochemical modelling. Using stable carbon isotopes, bulk rock parameters and petrographic analysis, the onset of OAE 2 has been investigated in detail. The high-resolution &#948;13C curve exhibits overall stable values around 3 &#8240; before the onset of the Plenus event. This background level is interrupted by three short-lived and small but significant negative carbon isotope excursions (CIEs) down to &#948;13C values of 2.5 &#8240;, 2.7 &#8240; and 1.9 &#8240;. Immediately before the main rise in the Plenus bed, a longer-lasting negative CIE down to 2.8 &#8240; is observed, preceding the large positive CIE of the OAE 2 to values of 5.2 &#8240; over 33 ka. Thereafter, the &#948;13C values decrease to 3.5 &#8240; over a period of approximately 130 ka. The results can be correlated with the lower-resolution data set of Voigt et al. (2008) but enable a more accurate characterization of the subtle features of the CIE and hence events before and during this time interval. Carbon cycle modelling with the modelling software SIMILE using a model based on Kump & Arthur (1999) reveals that the negative excursion before the Plenus bed can be explained by a massive volcanic pulse releasing of 0.95*1018 mol CO2 within 14 ka. This amount corresponds to only 81 % of the calculated volume of CO2 release during emplacement of the Caribbean LIP by Joo et al. (2020). In the model the volcanic exhalation increases atmospheric CO2 concentrations. This will increase global temperatures, intensify the hydrological cycle and thus increase nutrient input into the ocean, resulting in an expansion of the oxygen minimum zone, the development of anoxic conditions and an increase in the preservation potential for organic material. In the model enhanced primary productivity and organic matter preservation can be controlled by the implemented riverine phosphate input and the preservation factor for organic matter. For the positive anomaly, the riverine phosphate input must be nearly doubled (from 0.01 &#956;mol/kg PO4 to 0.019 &#956;mol/kg) for the period of the increasing &#948;13C values (app. 33 ka), with a concomitant rise of the preservation factor from 1 % to 2 %. This model scenario accurately reproduces the major features of the new high-resolution &#948;13C record over the onset of the OAE 2 CIE.

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