Latitudinal differences in the amplitude of the OAE-2 carbon isotopic excursion: pCO2 and paleoproductivity [Discussion paper]

A complete, well-preserved record of the Cenomanian/Turonian (C/T) Oceanic Anoxic Event 2 (OAE-2) was recovered from Demerara Rise in the southern North Atlantic Ocean (ODP site 1260). Across this interval, we determined changes in the stable carbon isotopic composition of sulfur-bound phytane (δ13Cphytane, a biomarker for photosynthetic algae. The δ13Cphytane record shows a positive excursion at the onset of the OAE-2 interval, with an unusually large amplitude (~7 ‰) compared to existing C/T proto-North Atlantic δ13Cphytane records (3–6 ‰). Overall, the amplitude of the excursion of δ13Cphytane decreases with latitude. Using reconstructed sea surface temperature (SST) gradients for the proto-North Atlantic, we investigated environmental factors influencing the latitudinal δ13Cphytane gradient. The observed gradient is best explained by high productivity at DSDP Site 367 and Tarfaya basin before OAE-2, which changed in overall high productivity throughout the proto-North Atlantic during OAE-2. During OAE-2, productivity at site 1260 and 603B was thus more comparable to the mid-latitude sites. Using these constraints as well as the SST and δ13Cphytane-records from Site 1260, we subsequently reconstructed pCO2 levels across the OAE-2 interval. Accordingly, pCO2 decreased from ca. 1750 to 900 ppm during OAE-2, consistent with enhanced organic matter burial resulting in lowering pCO2. Whereas the onset of OAE-2 coincided with increased pCO2, in line with a volcanic trigger for this event, the observed cooling within OAE-2 probably resulted from CO2 sequestration in black shales outcompeting CO2 input into the atmosphere. Together these results show that the ice-free Cretaceous world was sensitive to changes in pCO2 related to perturbations of the global carbon cycle

Metabarcoding Insights Into the Trophic Behavior and Identity of Intertidal Benthic Foraminifera

Foraminifera are ubiquitous marine protists with an important role in the benthic carbon cycle. However, morphological observations often fail to resolve their exact taxonomic placement and there is a lack of field studies on their particular trophic preferences. Here, we propose the application of metabarcoding as a tool for the elucidation of the in situ feeding behavior of benthic foraminifera, while also allowing the correct taxonomic assignment of the feeder, using the V9 region of the 18S (small subunit; SSU) rRNA gene. Living foraminiferal specimens were collected from two intertidal mudflats of the Wadden Sea and DNA was extracted from foraminiferal individuals and from the surrounding sediments. Molecular analysis allowed us to confirm that our foraminiferal specimens belong to three genetic types: Ammonia sp. T6, Elphidium sp. S5 and Haynesina sp. S16. Foraminiferal intracellular eukaryote communities reflected to an extent those of the surrounding sediments but at different relative abundances. Unlike sediment eukaryote communities, which were largely determined by the sampling site, foraminiferal intracellular eukaryote communities were driven by foraminiferal species, followed by sediment depth. Our data suggests that Ammonia sp. T6 can predate on metazoan classes, whereas Elphidium sp. S5 and Haynesina sp. S16 are more likely to ingest diatoms. These observations, alongside the use of metabarcoding in similar ecological studies, significantly contribute to our overall understanding of the ecological roles of these protists in intertidal benthic environments and their position and function in the benthic food webs.Peer reviewe

Effect of different seawater Mg2Â + concentrations on calcification in two benthic foraminifers

Magnesium, incorporated in foraminiferal calcite (Mg/CaCC), is used intensively to reconstruct past seawater temperatures but, in addition to temperature, the Mg/CaCC of foraminiferal tests also depends on the ratio of Mg and Ca in seawater (Mg/CaSW). The physiological mechanisms responsible for these proxy relationships are still unknown. This culture study investigates the impact of different seawater Mg2 + on calcification in two benthic foraminiferal species precipitating contrasting Mg/{CaCC}: Ammonia aomoriensis, producing low-Mg calcite and Amphistegina lessonii, producing intermediate-Mg calcite. Foraminiferal growth and test thickness were determined and, Mg/Ca was analyzed using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry ({LA}-{ICP}-{MS}). Results show that at present-day seawater Mg/{CaSW} of {\textasciitilde} 5, both species have highest growth rates, reflecting their adaptation to modern seawater element concentrations. Test thickness is not significantly affected by different Mg/{CaSW}. The relationship between Mg/{CaSW} and Mg/{CaCC} shows a distinct positive y-axis intercept, possibly reflecting at least two processes involved in foraminiferal biomineralization. The associated Mg partition ({DMg}) changes non-linearly with increasing Mg/{CaSW}, hence suggesting that the {DMg} is best described by an exponential function approaching an asymptote

Organic matter variations in the Northwestern Arabian Sea sediments

In this study composition of organic matter and its variations in marine sediments of northwestern Arabian sea has been discussed. This paper presents the geochemical analysis of organic carbon content, C/N and δ13 stable carbon isotope. The primary objective was to investigate the organic matter in sediments below an upwelling area. Undisturbed sediments (Piston core NIOP-486) of late Pleistocene time was collected during Netherlands Indian Ocean Program (NIOP-1992-93). The core NIOP-486 was raised from a depth of 2077 meters near the Owen ridge. This core records deposition history of last 200,000 years and includes 4 warm and 3 cold periods. The distribution of organic carbon content in studied core shows cyclicity during glacial and interglacial stages. The source of organic matter and variations in glacial/interglacial stages are discussed. C/N ratio and δ13 Corg isotope results are described to assess the relative proportions of terrestrial and marine organic matter

Element/Ca ratios in Nodosariida (Foraminifera) and their potential application for paleoenvironmental reconstructions

The chemical composition of foraminiferal shells is a well-known tool in paleoceanography to reconstruct past environments and climate. Their application is based on the relation between environmental variables and the concentration of elements incorporated or stable isotope fractionation during calcification. The vast majority of these so-called proxy relationships are based on the foraminiferal order of the Rotaliida, which, for example, encompasses all living planktonic species. However, there are more orders of foraminifera with calcifying members, some of which have fundamentally different biomineralization pathways, such as the Nodosariida, the Polymorphinida and the Vaginulinida. All these belong to the class of the Nodosariata and produce calcite shells, which may serve as carriers of paleoenvironmental and climate signals. The microstructures of these shells and overall morphology of these foraminifera strongly deviate from the Rotaliida, suggesting that their elemental and stable isotopic composition do not necessarily respond similarly to environmental parameters. A potential advantage of the Nodosariata is that they appear considerably earlier in the fossil record (Carboniferous) than the Rotaliida (Jurassic), thereby possibly extending the range of foraminifer-based paleoceanographic reconstructions considerably. To test the potential application of Nodosariata foraminifera as paleoproxies, we investigated incorporation of 5 elements in 11 species as a function of environmental parameters from a transect sampled in the Gulf of Mexico. Their element composition (B / Ca, Na / Ca, Mg / Ca, Sr / Ca and Ba / Ca) shows a distinct geochemical signature for these foraminifera, different to that of members of other foraminiferal orders. Results also show an increase in Mg / Ca values with increasing temperature, similar to that known for the Rotaliida, which suggest that Nodosariata shells might be useful for paleotemperature reconstructions. The difference in Mg / Ca-temperature calibration in Nodosariata compared to Rotaliida, with the large differences in their morphology, shell microstructures and overall geochemical composition, suggests that the Mg / Ca-to-temperature relationship is partly independent of the exact calcification mechanism. We compare Mg / Ca-temperature sensitivities across foraminiferal orders and describe a relationship between the average Mg / Ca and the sensitivity of the Mg / Ca-temperature calibration. For other elements, the variability across orders is smaller compared to that in Mg / Ca, which results in more similar El / Ca-environmental calibrations

Exploring foraminiferal Sr/Ca as a new carbonate system proxy

In present day paleoclimate research one of the most pressing challenges is the reconstruction of atmospheric CO2 concentrations. A variety of proxies for several components of the marine inorganic carbon system have been developed in this context (e.g. B isotopes, B/Ca, U/Ca) to allow reconstruction of past seawater pH, HCO3− and CO32− and thereby facilitate estimates of past atmospheric pCO2. Based on culture experiments using the benthic foraminifera Ammonia sp. we describe a positive correlation between Sr/Ca and the carbonate system, namely DIC/bicarbonate ion concentration. Foraminiferal Sr/Ca ratios provide potentially additional constraints on the carbonate system proxy, because the analysis of foraminiferal carbonate Sr/Ca is straightforward and not easily contaminated. Applying our calibration to a published dataset of paleo-Sr/Ca suggests the validity of Sr/Ca as a carbonate system proxy. Furthermore, we explore how our data can be used to advance conceptual understanding of the foraminiferal biomineralization mechanism

How dry was the Mediterranean during the Messinian salinity crisis?

The Messinian salinity crisis (MSC; 5.97\u20135.33 Ma) is an enigmatic episode of paleoceanographic change, when kilometers-thick evaporite units were deposited in the Mediterranean basin. It is generally accepted that during the MSC interval there was a dry climate in the Mediterranean region. It is difficult to assess how dry the climate was during the MSC because a modern analogue, in size and duration, is absent. Here we reconstruct hydrological changes in the Mediterranean basin during the three main MSC stages using excellently preserved biomarkers. We used the hydrogen isotopic composition of the long chain n-alkanes (\u3b4Dn-alkanes) to reconstruct the hydrological changes on the land adjacent to the Mediterranean Sea. Additionally, the \u3b4D of long-chain alkenones (\u3b4Dalkenones) is used to observe changes in the Mediterranean Sea water source. The \u3b4Dn-alkanes recorded during the deposition of Primary Lower Gypsum (stage 1) in Monte Tondo indicate a \u3b4D of the precipitation comparable to the present-day Mediterranean implying a similar hydrologic regime (indicated by experiments modelling the Miocene-Pliocene transition). Elevated \u3b4Dalkenones values from halite unit (stage 2) of the Realmonte mine are associated with kainite and giant polygons, consistent with presumably high evaporative conditions during halite deposition. The \u3b4Dn-alkanes recorded during the deposition of Upper Gypsum (stage 3) in Eraclea Minoa indicate a \u3b4Dprecipitation typical for much drier settings, similar to the Red Sea region. The relative contribution of the different alkenones from Eraclea Minoa is similar to the one observed in present-day marine settings suggesting that, during stage 3, connections to the open Ocean were likely maintained. However, the \u3b4Dalkenones records during deposition of the evaporites in Eraclea Minoa are similar to those synchronously registered in the Black Sea implying that a similar hydrologic regime, characterized by extended drought, covered large areas of southeastern Europe. Based on the \u3b4Dalkenones similarity and the Paratethys type of \ub4Lago Mare\ub4 fauna in the Mediterranean we speculate that the surface water during stage 3 was, at times, derived from the Black Sea

Bromine counts from XRF scanning as an estimate of the marine organic carbon content of sediment cores

XRF sediment core scanning technology is increasingly used to quantify sediment composition. The overall good correlation between biophilic halogen bromine (Br) and sedimentary total organic carbon (TOC) potentially allows the fast estimation of down core TOC profiles by XRF scanning. In order to test this approach we present data from the Arabian Sea and a Mediterranean brine basin, comparing XRF core scanning Br data with discrete sample TOC analyses. Overall, Br counts and TOC show a clear correlation, except when stable carbon isotope and C/N data indicate intervals characterized by enhanced input of terrestrial organic matter. Hence, solid phase Br is exclusively associated with marine organic matter (MOC) and can be used as a direct estimate of this parameter after a calibration is established. High pore water Br in the brine core steepens the Br/TOC correlation but after salt correction shows a nearly identical gradient to that of the Arabian Sea cor

Variation in methanotroph-related proxies in peat deposits from Misten Bog, Hautes-Fagnes, Belgium

Methane emissions from peat bogs are strongly reduced by aerobic methane oxidising bacteria (methanotrophs) living in association with Sphagnum spp. Field studies and laboratory experiments have revealed that, with increasing water level and temperature, methanotrophic activity increases. To gain a better understanding of how longer term changes in methanotrophic activity are reflected in methanotroph biomarkers, a peat record (0–100 cm) from the Hautes-Fagnes (Belgium) encompassing the past 1500 years, was analysed for methanotroph-specific intact bacteriohopanepolyols (BHPs) and the carbon isotopic composition of diploptene. A predominance of aminobacteriohopanetetrol (aminotetrol) over aminobacteriohopanepentol (aminopentol) indicated the prevalence of type II methanotrophs. Relatively high methanotrophic activity was indicated by all methanotroph markers between 20 and 45 cm depth, around the present oxic–anoxic boundary, most likely representing the currently active methanotrophic community. Comparing methanotrophic markers in the deeper part of the peat profile with environmental variables afforded, however, no clear correlation between change in water level and methanotrophic activity. This is potentially caused by a predominance of type II methanotrophs, a combination of sources for methanotrophic biomarkers or insufficient variation in climatic changes. A proposed way forward would include a study of a core covering a longer timescale, thereby involving greater variability