Abstract. CO 2 emissions are leading to an acidification of the oceans. Predicting marine community vulnerability towards acidification is difficult, as adaptation processes cannot be accounted for in most experimental studies. Naturally CO 2 enriched sites thus can serve as valuable proxies for future changes in community structure. Here we describe a natural analogue site in the Western Baltic Sea. Seawater pCO 2 in Kiel Fjord is elevated for large parts of the year due to upwelling of CO 2 rich waters. Peak pCO 2 values of >230 Pa (>2300 µatm) and pH NBS values of <7.5 are encountered during summer and autumn, average pCO 2 values are ∼70 Pa (∼700 µatm). In contrast to previously described naturally CO 2 enriched sites that have suggested a progressive displacement of calcifying auto-and heterotrophic species, the macrobenthic community in Kiel Fjord is dominated by calcifying invertebrates. We show that blue mussels from Kiel Fjord can maintain control rates of somatic and shell growth at a pCO 2 of 142 Pa (1400 µatm, pH NBS = 7.7). Juvenile mussel recruitment peaks during the summer months, when high water pCO 2 values of ∼100 Pa (∼1000 µatm) prevail. Our findings indicate that calcifying keystone species may be able to cope with surface ocean pH NBS values projected for the end of this century when food supply is sufficient. However, owing to non-linear synergistic effects of future acidification and upwelling of corrosive water, peak seawater pCO 2 in Kiel Fjord and many other productive estuarine habitats could increase to values >400 Pa (>4000 µatm). These changes will most likely affect calcification and recruitment, and increase external shell dissolution.

Widespread evidence of a +4-6-m sea-level highstand during the last interglacial period (Marine Isotope Stage 5e) has led to warnings that modern ice sheets will deteriorate owing to global warming and initiate a rise of similar magnitude by ad 2100 (ref. 1). The rate of this projected rise is based on ice-sheet melting simulations and downplays discoveries of more rapid ice loss. Knowing the rate at which sea level reached its highstand during the last interglacial period is fundamental in assessing if such rapid ice-loss processes could lead to future catastrophic sea-level rise. The best direct record of sea level during this highstand comes from well-dated fossil reefs in stable areas. However, this record lacks both reef-crest development up to the full highstand elevation, as inferred from widespread intertidal indicators at +6 m, and a detailed chronology, owing to the difficulty of replicating U-series ages on submillennial timescales. Here we present a complete reef-crest sequence for the last interglacial highstand and its U-series chronology from the stable northeast Yucatán peninsula, Mexico. We find that reef development during the highstand was punctuated by reef-crest demise at +3 m and back-stepping to +6 m. The abrupt demise of the lower-reef crest, but continuous accretion between the lower-lagoonal unit and the upper-reef crest, allows us to infer that this back-stepping occurred on an ecological timescale and was triggered by a 2-3-m jump in sea level. Using strictly reliable (230)Th ages of corals from the upper-reef crest, and improved stratigraphic screening of coral ages from other stable sites, we constrain this jump to have occurred approximately 121 kyr ago and conclude that it supports an episode of ice-sheet instability during the terminal phase of the last interglacial period.

We present strontium (Sr) isotope ratios that, unlike traditional 87 Sr/ 86 Sr data, are not normalized to a fixed 88 Sr/ 86 Sr ratio of 8.375209 (defined as d 88/86 Sr = 0 relative to NIST SRM 987). Instead, we correct for isotope fractionation during mass spectrometry with a 87 Sr-84 Sr double spike. This technique yields two independent ratios for 87 Sr/ 86 Sr and 88 Sr/ 86 Sr that are reported as ( 87 Sr/ 86 Sr*) and (d 88/86 Sr), respectively. The difference between the traditional radiogenic ( 87 Sr/ 86 Sr normalized to 88 Sr/ 86 Sr = 8.375209) and the new 87 Sr/ 86 Sr* values reflect natural mass-dependent isotope fractionation. In order to constrain glacial/interglacial changes in the marine Sr budget we compare the isotope composition of modern seawater (( 87 Sr/ 86 Sr*, d 88/86 Sr) Seawater ) and modern marine biogenic carbonates (( 87 Sr/ 86 Sr*, d 88/86 Sr) Carbonates ) with the corresponding values of river waters (( 87 Sr/ 86 Sr*, d 88/86 Sr) River ) and hydrothermal solutions (( 87 Sr/ 86 Sr*, d 88/86 Sr) HydEnd ) in a triple isotope plot. The measured ( 87 Sr/ 86 Sr*, d 88/86 Sr) River values of selected rivers that together account for $18% of the global Sr discharge yield a Sr flux-weighted mean of (0.7114 (8), 0.315(8)&). The average ( 87 Sr/ 86 Sr*, d 88/86 Sr) HydEnd values for hydrothermal solutions from the Atlantic Ocean are (0.7045(5), 0.27(3)&). In contrast, the ( 87 Sr/ 86 Sr*, d 88/86 Sr) Carbonates values representing the marine Sr output are (0.70926(2), 0.21(2)&). We estimate the modern Sr isotope composition of the sources at (0.7106(8), 0.310(8)&).The difference between the estimated ( 87 Sr/ 86 Sr*, d 88/86 Sr) input and ( 87 Sr/ 86 Sr*, d 88/86 Sr) output values reflects isotope disequilibrium with respect to Sr inputs and outputs. In contrast to the modern ocean, isotope equilibrium between inputs and outputs during the last glacial maximum (10-30 ka before present) can be explained by invoking three times higher Sr inputs from a uniquely "glacial" source: weathering of shelf carbonates exposed at low sea levels. Our data are also consistent with the "weathering peak" hypothesis that invokes enhanced Sr inputs resulting from weathering of postglacial exposure of abundant fine-grained material.

The uptake of anthropogenic emission of carbon dioxide is resulting in a lowering of the carbonate saturation state and a drop in ocean pH. Understanding how marine calcifying organisms such as coralline algae may acclimatize to ocean acidification is important to understand their survival over the coming century. We present the first long-term perturbation experiment on the cold-water coralline algae, which are important marine calcifiers in the benthic ecosystems particularly at the higher latitudes. Lithothamnion glaciale, after three months incubation, continued to calcify even in undersaturated conditions with a significant trend towards lower growth rates with increasing pCO2 . However, the major changes in the ultra-structure occur by 589 μatm (i.e. in saturated waters). Finite element models of the algae grown at these heightened levels show an increase in the total strain energy of nearly an order of magnitude and an uneven distribution of the stress inside the skeleton when subjected to similar loads as algae grown at ambient levels. This weakening of the structure is likely to reduce the ability of the alga to resist boring by predators and wave energy with severe consequences to the benthic community structure in the immediate future (50 years).

[1] Stable strontium isotopes (here 88 Sr/ 86 Sr) are introduced as a new member of the nontraditional stable isotopes. We have developed a bracketing standard method for the determination of d 88/86 Sr using an AXIOM MC-ICP-MS and normalizing to strontium SRM NBS987. For individual measurements the external reproducibility is better than about 25 ppm (1s RSD). For the IAPSO seawater standard a d 88/86 Sr value of 0. 381 ± 0.010% (2SEM) was determined. For the first time a temperature-dependent strontium isotope fractionation during calcium carbonate precipitation could be shown. Aragonite samples inorganically precipitated under temperature control between 10 and 50°C revealed a d 88/86 Sr/ temperature dependency of 0.0054(5)%/°C (R 2 = 0.987). In contrast, for natural coral samples (Pavona clavus) from a proxy calibration study (23 to 27°C) we did determine 0.033(5)%/°C (R 2 = 0.955). The processes causing this sixfold stronger temperature dependency for the natural coral samples have to be studied in more detail in future studies. In a first approach the different slopes can be interpreted as effects of kinetic fractionation of strontium ions with or without a hydrate shell of 22 to 29 water molecules.

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Strontium isotopes in various marine carbonates were determined using an ''AXIOM'' MC-ICP-MS in combination with a NewWave UP193 laser ablation unit. Using a modified measurement and data reduction strategy, an external reproducibility of 87 Sr/ 86 Sr ratios in carbonates of about 19 ppm (RSD) was achieved. For recent and sub-recent marine carbonates a mean radiogenic strontium isotope ratio 87 Sr/ 86 Sr of 0.709170 AE 0.000007 (2SE) was determined, which agrees well with the value of 0.7091741 AE 0.0000024 (2SE) reported for modern sea water (J.

The North Atlantic Oscillation is the dominant atmospheric pressure mode in the North Atlantic region and affects winter temperature and precipitation in the Mediterranean, northwest Europe, Greenland, and Asia1. The index1 that describes the sea-level pressure difference between Iceland and the Azores is correlated with a dipole precipitation pattern over northwest Europe and northwest Africa. How the North Atlantic Oscillation will develop as the Greenland ice sheet melts is unclear2. A potential past analogue is the early Holocene, during which melting ice sheets around the North Atlantic3, 4 freshened surface waters, affecting the strength of the meridional overturning circulation5. Here we present a Holocene rainfall record from northwest Africa based on speleothem δ18O and compare it against a speleothem-based rainfall record from Europe6. The two records are positively correlated during the early Holocene, followed by a shift to an anti-correlation, similar to the modern record, during the mid-Holocene. On the basis of our simulations with an Earth system model, we suggest the shift to the anti-correlation reflects a large-scale atmospheric and oceanic reorganization in response to the demise of the Laurentide ice sheet and a strong reduction of meltwater flux to the North Atlantic, pointing to a potential sensitivity of the North Atlantic Oscillation to the melting of ice sheets