Stable isotope composition of Cretaceous benthic foraminifera: biological and environmental effects

Abstrac

Braving the extremes: foraminifera document changes in climate-induced and anthropogenic stress in Wadden Sea salt marshes

Tidal wetlands are highly dynamic ecosystems that are susceptible to changes in sea level and flooding from storm surges. Among them, salt marshes play a key role in coastal protection as they contribute to wave attenuation through their regulating ecosystem services, thereby promoting sediment deposition and shoreline stabilization. However, the resilience of salt marshes, particularly those that have been modified and cultivated for centuries, is questionable in the face of accelerated sea-level rise (SLR) and increasing run-up heights of storm surges. In this context, this study aims to investigate the historical foraminiferal records of two sedimentary salt marsh archives from the Wadden Sea area (Dithmarschen and North Frisia, Germany) that have been modified to varying degrees by human management activities over the last century. The foraminiferal records document how physico-chemical traits of salt marshes of the central Wadden Sea have responded to storm tide inundation over the last century, providing information about salt marsh stability and vulnerability. Abnormally grown tests of the salt marsh indicator species Entzia macrescens increased in number between 1950 CE and the late 1980s, indicating the concurrent increase of environmental stress caused by the effects of times of increased salt marsh flooding. These trends can be linked to observations of amplified North Sea storm surges, corroborating that salt marsh ecosystems respond to changing climate conditions. Differences in the number of abnormal foraminifera between the studied salt marshes suggest a particularly high vulnerability of intensively human-modified coastal wetland ecosystems to amplified storm climate conditions

Messinian productivity changes in the northeastern Atlantic and their relationship to the closure of the Atlantic-Mediterranean gateway: implications for Neogene palaeoclimate and palaeoceanography.

The stable isotope composition of planktic and benthic foraminifera and the distribution of selected benthic foraminiferal species from a Messinian record of the lower Guadalquivir Basin, northeastern Atlantic Ocean, show that regional productivity changes were linked to glacioeustatic fluctuations. Glacial periods were characterized by poorly ventilated bottom waters as a result of weak Atlantic Meridional Overturning Circulation (AMOC), and by phases of high productivity related to intensified upwelling. In contrast, wellventilated bottom waters owing to strong AMOC, the presence of degraded organic matter in the upper slope, and high input of degraded terrestrial organic matter derived from fluvial discharge to the outer shelf were recorded during interglacial periods. Before closure of the adjacent Guadalhorce Corridor at 6.18 Ma, which was the final active Betic Atlantic-Mediterranean gateway, the study area was alternately influenced by wellventilated Mediterranean Outflow Water (MOW) and poorly ventilated Atlantic Upwelled Water (AUW). Following closure of the corridor, cessation of the MOW reduced the AMOC and promoted glacial conditions in the northern hemisphere, resulting in the establishment of local upwelling cells

Impact of restriction of the Atlantic-Mediterranean gateway on the Mediterranean Outflow Water and eastern Atlantic circulation during the Messinian

Messinian foraminiferal stable oxygen and carbon isotopes of the Montemayor-1 core (Guadalquivir Basin, SW Spain) have been investigated. This record is exceptional to study the Mediterranean Outflow Water (MOW) impact on the Atlantic meridional overturning circulation (AMOC) and global climate during the Messinian because the core is near the Guadalhorce Corridor, the last Betic gateway to be closed during the early Messinian. Our results allow dating accurately its closure at 6.18 Ma. Constant benthic d18O values, high difference between benthic and planktonic d18O, and low sedimentation rates before 6.18 Ma indicate the presence of a two-layer water column, with bottom winnowing due to an enhanced Mediterranean outflow current. The enhanced contribution of dense MOW to the North Atlantic Ocean likely fostered the formation of North Atlantic Deep Water (NADW). After 6.18 Ma, benthic d18O values parallel that of the global glacioeustatic curve, the difference between benthic and planktonic d18O is low, and sedimentation rates considerably increased. This indicates a good vertical mixing of the water column, interruption of the MOW, and a dominant glacioeustatic control on the isotopic signatures. According to the role of MOW in the modern Atlantic thermohaline circulation, the reduction of the MOW after the closure of the Guadalhorce Corridor might have resulted in a decreased NADW formation rate between 6.0 and 5.5 Ma weakening the AMOC and promoting northern hemisphere cooling. After the Gibraltar Strait opening, the restoration of the MOW and related salt export from the Mediterranean could have promoted an enhanced NADW formation

Assessing the long‐term carbon‐sequestration potential of the semi‐natural salt marshes in the European Wadden Sea

Salt marshes and other blue carbon ecosystems have been increasingly recognized for their carbon (C)‐sink function. Yet, an improved assessment of organic carbon (OC) stocks and C‐sequestration rates is still required to include blue C in C‐crediting programs. Particularly, factors inducing variability in the permanence of sequestration and allochthonous contributions to soil OC stocks require an improved understanding. This study evaluates the potential for long‐term C sequestration in the semi‐natural salt marshes of the European Wadden Sea (WS), conducting deep (1.3 m) down‐core OC‐density assessments in sites with known site histories and accretion records. Because these young marshes have developed from tidal‐flat ecosystems and have undergone rapid succession during the last 80–120 yr, the identification of different ecosystem stages down‐core was crucial to interpret possible changes in OC density. This was conducted based on the down‐core distribution of different foraminiferal taxa and grain sizes. Comparisons of historic and recent accretion rates were conducted to understand possible effects of accretion rate on down‐core changes in OC density. δ13C in OC was used to assess the origin of accumulated OC (autochthonous vs. allochthonous sources). We show that large amounts of short‐term accumulated OC are lost down‐core in the well‐aerated marsh soils of the WS region and thus emphasize the importance of deep sampling to avoid overestimation of C sequestration. Despite steep declines in OC‐density down‐core, minimum values of OC density in the salt‐marsh soils were considerably higher than those of the former tidal‐flat sediments that the marshes were converted from, illustrating the greater C‐sequestration potential of the vegetated ecosystem. However, our data also suggest that marine‐derived allochthonous OC makes up a large fraction of the effectively, long‐term preserved OC stock, whereas atmospheric CO2 removal by marsh vegetation contributes relatively little. The implication of this finding for C‐crediting approaches in blue C ecosystems has yet to be clarified

Messinian paleoenvironmental evolution in the lower Guadalquivir Basin (SW Spain) based on benthic foraminifera.

Benthic foraminiferal assemblages of a drill core from the lower Guadalquivir Basin (northern Gulf of Cádiz, SW Spain) have been analyzed in order to reconstruct the paleoenvironmental evolution in the vicinity of the Betic seaways during the Messinian. The core consists of marine sediments ranging from the latest Tortonian to the early Pliocene. Changes in the abundance of certain marker species, planktonic/benthic ratio (P/B ratio), paleodepth estimated with a transfer function, content of sand grains and presence of glauconitic layers indicate a complete transgressive-regressive sea-level cycle from the bottom to the top of the section. An abrupt sea-level rise, from inner-middle shelf to middle slope, is recorded at the lowermost part of the core (latest Tortonian-earliest Messinian), followed by a relatively rapid shallowing from middle slope to outer shelf. Magnetobiostratigraphic data show that this sea-level fall postdates the onset of the Messinian salinity crisis (MSC) in the Mediterranean. Finally, the early Pliocene deposits are interpreted as inner-middle shelf. Changes in the benthic foraminiferal assemblages through the core are mainly controlled by the trophic conditions, specifically by the quantity and quality of the organic matter reaching the sea floor. The upper slope and part of the outer shelf assemblages are highly diverse and dominated by shallow infaunal species, indicating a generallymesotrophic environment with moderate oxygenation. These environments have likely been affected by repeated upwelling events, documented by increased abundance of Uvigerina peregrina s.l., an opportunistic species thriving in environmentswith enhanced labile organic matter supply. The assemblages of the transitional interval between upper slope to outer shelf, and of the outer shelf are generally characterized by a relatively low diversity and epifaunal-shallow infaunal taxa, indicating oligotrophic and well-oxygenated conditions. The innermiddle shelf assemblages are characterized by very lowdiversity and dominance of intermediate to deep infaunal taxa, suggesting an eutrophic environmentwith lowoxygen content. These assemblages are dominated by Nonion fabum and Bulimina elongata, two taxa that are able to feed from continental low-quality organicmatter,most likely derived from river run-off. The paleoenviromental evolution on the Atlantic side of Betic and Rifian seaways is similar during the Messinian, with a Messinian continuous sea-level lowering driven by regional tectonic uplift and upwelling-related waters reaching the upper slope. This study will further contribute to understand the role of tectonics on the sea-level changes as well as on the closure of the Atlantic-Mediterranean gateways that led to the MSC, and on the paleoceanography on the Atlantic sides of these corridors

Glacial-interglacial changes in bottom-water oxygen content on the Portuguese margin

During the last and penultimate glacial maxima, atmospheric CO2 concentrations were lower than present, possibly in part because of increased storage of respired carbon in the deep oceans. The amount of respired carbon present in a water mass can be calculated from its oxygen content through apparent oxygen utilization; the oxygen content can in turn be calculated from the carbon isotope gradient within the sediment column. Here we analyse the shells of benthic foraminifera occurring at the sediment surface and the oxic/anoxic interface on the Portuguese Margin to reconstruct the carbon isotope gradient and hence bottom-water oxygenation over the past 150,000 years. We find that bottom-water oxygen concentrations were 45 and 65 μmol kg−1 lower than present during the last and penultimate glacial maxima, respectively. We calculate that concentrations of remineralized organic carbon were at least twice as high as today during the glacial maxima. We attribute these changes to decreased ventilation linked to a reorganization of ocean circulation and a strengthened global biological pump. If the respired carbon pool was of a similar size throughout the entire glacial deep Atlantic basin, then this sink could account for 15 and 20 per cent of the glacial PCO2 drawdown during the last and penultimate glacial maxima

Glacioeustatic control on the origin and cessation of the Messinian salinity crisis.

The desiccation of the Mediterranean during the Messinian salinity crisis (MSC) is one of the most intriguing geological events of recent Earth history. However, the timing of its onset and end, as well as themechanisms involved remain controversial.We present a novel approach to these questions by examining theMSC fromthe Atlantic, but close to theGibraltar Arc, analysing the completeMessinian record of theMontemayor-1 core of theGuadalquivir Basin (SWSpain). Flexural backstripping analysis shows a tectonic uplift trend that would have reduced the depth of the Rifian Corridors considerably. Nonetheless, the rate of tectonic upliftwas insufficient to account for the close up of the corridors. At 5.97 Ma, a global cooling and associated glacioeustatic sea-level drop, estimated in 60 m, is observed. Thiswould have been sufficient to restrict the Rifian Corridors and to trigger theMSC. The later flooding of the Mediterranean occurred during a sea-level rise associated with global warming during a stable tectonic period. We postulate a two-step flooding event: 1) A glacioeustatic sea-level rise during interglacial stage TG 11 (5.52 Ma) led to subtropical Atlantic waters entering the west-central Mediterranean through pathways south of the Gibraltar Strait, probably the Rifian Corridors. 2) A global sea-level drop at 5.4 Ma, thatmight have favoured intensification of regressive fluvial erosion in the Gibraltar threshold, along with the subsequent global sea-level rise would have generated the Gibraltar Strait leading to complete Mediterranean refilling during the earliest Pliocene