Jacek Raddatz

Sea surface and subsurface circulation dynamics off equatorial Peru during the last ~17 kyr

The complex deglacial to Holocene oceanographic development in the Gulf of Guayaquil (Eastern Equatorial Pacific) is reconstructed for sea surface and subsurface ocean levels from (isotope) geochemical proxies based on marine sediment cores. At sea surface, southern sourced Cold Coastal Water and tropical Equatorial Surface Water/Tropical Surface Water are intimately related. In particular since ~10 ka, independent sea surface temperature proxies capturing different seasons emphasize the growing seasonal contrast in the Gulf of Guayaquil, which is in contrast to ocean areas further offshore. Cold Coastal Water became rapidly present in the Gulf of Guayaquil during the austral winter season in line with the strengthening of the Southeast Trades, while coastal upwelling off Peru gradually intensified and expanded northward in response to a seasonally changing atmospheric circulation pattern affecting the core locations intensively since 4 ka BP. Equatorial Surface Water, instead, was displaced and Tropical Surface Water moved northward together with the Equatorial Front. At subsurface, the presence of Equatorial Under Current-sourced Equatorial Subsurface Water was continuously growing, prominently since ~10–8 ka B.P. During Heinrich Stadial 1 and large parts of the Bolling/Allerod, and similarly during short Holocene time intervals at ~5.1–4 ka B.P. and ~1.5–0.5 ka B.P., the admixture of Equatorial Subsurface Water was reduced in response to both short-term weakening of Equatorial Under Current strength from the northwest and emplacement by tropical Equatorial Surface Water, considerably warming the uppermost ocean layers.

Paleoseawater density reconstruction and its implication for cold-water coral carbonate mounds in the northeast Atlantic through time

Carbonate buildups and mounds are impressive biogenic structures throughout Earth history. In the recent NE Atlantic, cold-water coral (CWC) reefs form giant carbonate mounds of up to 300 m of elevation. The expansion of these coral carbonate mounds is paced by climatic changes during the past 2.7 Myr. Environmental control on their development is directly linked to controls on its main constructors, the reef-building CWCs. Seawater density has been identified as one of the main controlling parameter of CWC growth in the NE Atlantic. One possibility is the formation of a pycnocline above the carbonate mounds, which is increasing the hydrodynamic regime, supporting elevated food supply, and possibly facilitating the distribution of coral larvae. The potential to reconstruct past seawater densities from stable oxygen isotopes of benthic foraminifera has been further developed: a regional equation gives reliable results for three different settings, peak interglacials (e.g., Holocene), peak glacials (e.g., Last Glacial Maximum), and intermediate setting (between the two extremes). Seawater densities are reconstructed for two different NE Atlantic CWC carbonate mounds in the Porcupine Seabight indicating that the development of carbonate mounds is predominantly found at a seawater density range between 27.3 and 27.7 kg m−3 (σΘ notation). Comparable to recent conditions, we interpret the reconstructed density range as a pycnocline serving as boundary layer, on which currents develop, carrying nutrition and possibly coral larvae. The close correlation of CWC reef growth with reconstructed seawater densities through the Pleistocene highlights the importance of pycnoclines and intermediate water mass dynamics.

Bioluminescence in deep-sea isidid gorgonians from the Cape Verde archipelago

Cold-water corals, and in particular numerous gorgonian species, occur abundantly on the deep slopes of the Cape Verde archipelago (Fig. 1a). Among them, the isidid gorgonian genus Keratoisis occurred frequently. A living Keratoisis sp. was ROV collected (KIEL 6000) from 3,052 m (16 42.3¢N, 25 34.9¢W) in the Charles Darwin Volcanic Field (1b) during METEOR cruise M80/3. A strong luminescence was accidentally observed when this bamboo coral arrived on deck in the early evening hours just before sunset. The entire stem and branch tissue showed a dull blue luminescence. Additionally, when touched, it emitted a very strong blue light (Fig. 1c) that persisted for a few seconds. Coral tissue lit up strongest and flash like at the point of stimulation, and the illumination spread in a wave across the coenenchyme of the distal branches. The most intense light emission originated from the non-retractile sclerite-rich feeding polyps and remained visible for several minutes before it slowly faded. This phenomenon could be reproduced several times within hours. Luminescence in octocorals has been observed in the alcyonarian Anthomastus sp., as well as in isidid gorgonians (Isidella, Keratoisis, and Lepidisis), primnoid gorgonians (Primnoisis and Thouarella), and in Iridigorgia and Acanthogorgia (Herring 1987). Muzik (1978) documented bioluminescence in the isidid gorgonian Lepidisis olapa off Hawaii, and Etnoyer (2008) mentioned luminescent capabilities for Isidella tentaculum from the northeast Pacific. Just recently, bioluminescence was reported for Keratoisis flexibilis and for the zoanthid Gerardia sp. from the Gulf of Mexico (http://oceanexplorer.noaa.gov/explorations/09bioluminescence/). Likely due to the scarce availability of direct deep-sea sampling and observation, there are no further Atlantic records for bioluminescence in the Keratoisidinae outside the Gulf of Mexico. Our additional observations support that bioluminescence in Keratoisis and in other deep-sea gorgonians is rather common and deserves detailed in situ observations.

Assessing kinetic fractionation in brachiopod calcite using clumped isotopes

Brachiopod shells are the most widely used geological archive for the reconstruction of the temperature and the oxygen isotope composition of Phanerozoic seawater. However, it is not conclusive whether brachiopods precipitate their shells in thermodynamic equilibrium. In this study, we investigated the potential impact of kinetic controls on the isotope composition of modern brachiopods by measuring the oxygen and clumped isotope compositions of their shells. Our results show that clumped and oxygen isotope compositions depart from thermodynamic equilibrium due to growth rate-induced kinetic effects. These departures are in line with incomplete hydration and hydroxylation of dissolved CO2. These findings imply that the determination of taxon-specific growth rates alongside clumped and bulk oxygen isotope analyses is essential to ensure accurate estimates of past ocean temperatures and seawater oxygen isotope compositions from brachiopods.

Environmental constraints on Holocene cold-water coral reef growth off Norway: Insights from a multiproxy approach

High-latitude cold-water coral (CWC) reefs are particularly susceptible due to enhanced CO2 uptake in these regions. Using precisely dated (U/Th) CWCs (Lophelia pertusa) retrieved during research cruise POS 391 (Lopphavet 70.6°N, Oslofjord 59°N) we applied boron isotopes (δ11B), Ba/Ca, Li/Mg and U/Ca ratios to reconstruct the environmental boundary conditions of CWC reef growth. The sedimentary record from these CWC reefs reveals a lack of corals between ∼ 6.4 and 4.8 ka. The question remains if this phenomenon is related to changes in the carbonate system or other causes. The initial postglacial setting had elevated Ba/Ca ratios, indicative of meltwater fluxes showing a decreasing trend towards cessation at 6.4 ka with a oscillation pattern similar to continental glacier fluctuations. Downcore U/Ca ratios reveal an increasing trend, which is outside the range of modern U/Ca variability in L. pertusa, suggesting changes of seawater pH near 6.4 ka. The reconstructed BWT at Lopphavet reveals a striking similarity to Barent Sea-Surface and sub-Sea-Surface-Temperature records. We infer that meltwater pulses weakened the North Atlantic Current system resulting in southward advances of cold and CO2 rich Arctic waters. A corresponding shift in the δ11B record from ∼ 25.0‰ to ∼ 27.0 ‰ probably implies enhanced pH-up regulation of the CWCs due to the higher pCO2 concentrations of ambient seawater, which hastened Mid-Holocene CWC reef decline on the Norwegian Margin.