Impact of changing carbonate chemistry, temperature, and salinity on growth and test degradation of the benthic foraminifer Ammonia aomoriensis

The present study investigated the combined effects of ocean acidification, temperature, and salinity on growth and test degradation of Ammonia aomoriensis. This species is one of the dominant benthic foraminifera in near-coastal habitats of the southwestern Baltic Sea that can be particularly sensitive to changes in seawater carbonate chemistry. To assess potential responses to ocean acidification and climate change, we performed a fully crossed experiment involving three temperatures (8, 13, and 18°C), three salinities (15, 20, and 25) and four pCO2 levels (566, 1195, 2108, and 3843 µatm) for six weeks. Our results highlight a sensitive response of A. aomoriensis to undersaturated seawater with respect to calcite. The specimens continued to grow and increase their test diameter in treatments with pCO2 <1200 µatm, when Omega calc >1. Growth rates declined when pCO2 exceeded 1200 µatm (Omega calc <1). A significant reduction in test diameter and number of tests due to dissolution was observed below a critical Omega calc of 0.5. Elevated temperature (18°C) led to increased Omega calc, larger test diameter, and lower test degradation. Maximal growth was observed at 18°C. No significant relationship was observed between salinity and test growth. Lowered and undersaturated Omega calc, which results from increasing pCO2 in bottom waters, may cause a significant future decline of the population density of A. aomoriensis in its natural environment. At the same time, this effect might be partially compensated by temperature rise due to global warming.

TEX86H proxy and BIT index of sediment core GeoB9528-3 and reconstructed temperature data from NW Africa surface sediments

Changes in the strength of Atlantic meridional overturning circulation (AMOC) are known to have profound impacts on global climate. Coupled modelling studies have suggested that, on annual to multi-decadal time scales, a slowdown of AMOC causes a deepening of the thermocline in the tropical Atlantic. However, this process has been poorly constrained by sedimentary geochemical records. Here, we reconstruct surface (UK'37 Index) and thermocline (TEX86H) water temperatures from the Guinea Plateau Margin (Eastern tropical Atlantic) over the last two glacial-interglacial cycles (~ 192 kyr). These paleotemperature records show that periods of reduced AMOC, as indicated by the d13 C benthic foraminiferal record from the same core, coincide with a reduction in the near-surface vertical temperature gradient, demonstrating for the first time that AMOC-induced tropical Atlantic thermocline adjustment exists on longer, millennial time scales. Modelling results support the interpretation of the geochemical records and show that thermocline adjustment is particularly pronounced in the eastern tropical Atlantic. Thus, variations in AMOC strength appear to be an important driver of the thermocline structure in the tropical Atlantic from annual to multi-millennial time scales.

Sea-surface temperature reconstruction for sediment core GeoB1023-5

Sediment core GeoB 1023-5 from the eastern South Atlantic was investigated at high temporal resolution for variations of sea-surface temperature (SST) during the past 22 kyr, using the alkenone (UK'37) method. SSTs increased by 3.5°C from about 18°C during the Last Ice Age (21±2 cal kyr BP) to about 21.5°C at 14.5 cal kyr BP. This warming trend associated with the deglaciation phase was followed by a cooling event with lowest SSTs near 20°C, persisting for about 1000 years between 13 and 12 cal kyr BP. The SSTs then continued to increase to about 22.5°C at the Holocene climatic optimum at 7 cal kyr BP, and decreased again during the Late Holocene to a core-top value of 19.8°C that is comparable to modern annual mean SST values. When compared with alkenone SST records from the eastern North Atlantic, our SST record indicates continuous warming throughout the deglaciation phase in the Benguela Current, while its northern counterpart, the Canary Current, experienced prominent cooling during 'Heinrich Event 1' (H1). On the other hand, for the time period corresponding to the 'Younger Dryas' (YD) cooling event, the Benguela SST record exhibits a cold-temperature interval that corresponds to that observed in the eastern North Atlantic SST records. This observation suggests that interhemispheric climate response in Atlantic eastern boundary current systems was different with respect to the two abrupt climate events associated with Termination I. For the H1, the eastern South Atlantic SST record strongly supports the hypothesis that an 'anti-phase' thermal behavior in South Atlantic surface waters was forced by the slowdown of the North Atlantic Deep Water formation during cold spells in the North Atlantic. In contrast, the abrupt cooling in the eastern South Atlantic coincident with the YD period was probably induced by more vigorous global atmospheric circulation, enhancing the upwelling intensity in both eastern boundary current systems. This atmospheric control may have overridden any effect caused by changes in thermohaline circulation on the South Atlantic SSTs during the YD, which leads to the assumption that the thermohaline circulation was already much closer to its interglacial mode during the YD than during the H1.