ALBERT

Description: The rising temperature of the world’s oceans is affecting coral reef ecosystems by increasing the frequency and severity of bleaching and mortality events. The susceptibility of corals to temperature stress varies on local and regional scales. Insights into potential controlling parameters are hampered by a lack of long term in situ data in most coral reef environments and sea surface temperature (SST) products often do not resolve reef-scale variations. Here we use 42 years (1970–2012) of coral Sr/Ca data to reconstruct seasonal- to decadal-scale SST variations in two adjacent but distinct reef environments at Little Cayman, Cayman Islands. Our results indicate that two massive Diploria strigosa corals growing in the lagoon and in the fore reef responded differently to past warming events. Coral Sr/Ca data from the shallow lagoon successfully record high summer temperatures confirmed by in situ observations (〉338C). Surprisingly, coral Sr/Ca from the deeper fore reef is strongly affected by thermal stress events, although seasonal temperature extremes and mean SSTs at this site are reduced compared to the lagoon. The shallow lagoon coral showed decadal variations in Sr/Ca, supposedly related to the modulation of lagoonal temperature through varying tidal water exchange, influenced by the 18.6 year lunar nodal cycle. Our results show that reef-scale SST variability can be much larger than suggested by satellite SST measurements. Thus, using coral SST proxy records from different reef zones combined with in situ observations will improve conservation programs that are developed to monitor and predict potential thermal stress on coral reefs.

Description: Here we present the first record of Sr/Ca variability in a massive Porites lutea coral from the Lakshadweep Archipelago, Arabian Sea. The annual mean sea surface temperature (SST) in this region and the surrounding areas has increased steadily in the recent past. During some major El Nino events, SSTs are even higher, imposing additional thermal-stress on corals, episodically leading to coral bleaching. We infer from the coral-Sr/Ca record (1981-2008) that during some of these events high and persistent SSTs lead to a dampening of the temperature signal in coral-Sr/Ca, impairing the coral's ability to record full scale warming. Thus, coral-Sr/Ca may provide a history of past El Nino Southern-Oscillation (ENSO) induced thermal-stress episodes, which are a recurrent feature also seen in cross-spectral analysis between coral-Sr/Ca and the Nino3.4 index. Despite the impact of episodical thermal-stress during major El Nino events, our coral proxy faithfully records the seasonal monsoon-induced summer cooling on the order of approximate to 2.3 degrees C. Calibration of coral-Sr/Ca with instrumental grid-SST data shows significant correlation to regional SST and monsoon variability. Hence, massive Porites corals of this region are highly valuable archives for reconstructing long-term changes in SST, strongly influenced by monsoon variability on seasonal scales. More importantly, our data show that this site with increasing SST is an ideal location for testing the future effects of the projected anthropogenic SST increase on coral reefs that are already under thermal-stress worldwide.

Description: The Atlantis II Deep of the Red Sea hosts the largest known hydrothermal ore deposit on the ocean floor and the only modern analog of brine pool-type metal deposition. The deposit consists mainly of chemical-clastic sediments with input from basin-scale hydrothermal and detrital sources. A characteristic feature is the millimeter-scale layering of the sediments, which bears a strong resemblance to banded iron formation (BIF). Quantitative assessment of the mineralogy based on relogging of archived cores, detailed petrography, and sequential leaching experiments shows that Fe-(oxy)hydroxides, hydrothermal carbonates, sulfides, and authigenic clays are the main “ore” minerals. Mn-oxides were mainly deposited when the brine pool was more oxidized than it is today, but detailed logging shows that Fe-deposition and Mn-deposition also alternated at the scale of individual laminae, reflecting short-term fluctuations in the Lower Brine. Previous studies underestimated the importance of nonsulfide metal-bearing components, which formed by metal adsorption onto poorly crystalline Si-Fe-OOH particles. During diagenesis, the crystallinity of all phases increased, and the fine layering of the sediment was enhanced. Within a few meters of burial (corresponding to a few thousand years of deposition), biogenic (Ca)-carbonate was dissolved, manganosiderite formed, and metals originally in poorly crystalline phases or in pore water were incorporated into diagenetic sulfides, clays, and Fe-oxides. Permeable layers with abundant radiolarian tests were the focus for late-stage hydrothermal alteration and replacement, including deposition of amorphous silica and enrichment in elements such as Ba and Au.

Description: We present geological observations and geochemical data for the youngest volcanic features on the slow-spreading Mid-Atlantic Ridge at 8°48'S that shows seismic evidence for a thickened crust and excess magma formation. Young lava flows with high sonar reflectivity cover about 14 km2 in the axial rift and were probably erupted from two axial volcanic ridges each of about 3 km in length. Three different lava units occur along an about 11 km long portion of the ridge, and lavas from the northern axial volcanic ridge differ from those of the southern axial volcanic ridge and surrounding lava flows. Basalts from the axial rift flanks and from a pillow mound within the young flows are more incompatible element depleted than those from the young volcanic field. Lavas from this volcanic area have 226Ra-230Th disequilibria model ages of 1,000 and 4,000 years whereas the older lavas from the rift flank and the pillow mound, but also some of the lava field, are older than 8,000 years. Glasses from the northern and southern ends of the southern lava unit indicate up to 100°C cooler magma temperatures than in the center and increased assimilation of hydrothermally altered material. The compositional heterogeneity on a scale of 3 km suggests small magma batches rising vertically from the mantle to the surface without significant lateral flow and mixing. The observations on the 8°48'S lava field support the model of low frequency eruptions from single ascending magma batches that has been developed for slow-spreading ridges.

Description: [1] The Integrated Ocean Drilling Program (IODP) Expedition 310 recovered drill cores from the drowned reefs around the island of Tahiti (17°40′S, 149°30′W), many of which contained samples of massive corals from the genus Porites. Herein we report on one well-preserved fossil coral sample: a 13.6 cm long Porites sp. dated by uranium series techniques at 9523 ± 33 years. Monthly δ18O and Sr/Ca determinations reveal nine clear and robust annual cycles. Coral δ18O and Sr/Ca determinations estimate a mean temperature of ∼24.3°C (∼3.2°C colder than modern) for Tahiti at 9.5 ka; however, this estimate is viewed with caution since potential sources of cold bias in coral geochemistry remain to be resolved. The interannual variability in coral δ18O is similar between the 9.5 ka coral record and a modern record from nearby Moorea. The seasonal cycle in coral Sr/Ca is approximately the same or greater in the 9.5 ka coral record than in modern coral records from Tahiti. Paired analysis of coral δ18O and Sr/Ca indicates cold/wet (warm/dry) interannual anomalies, opposite from those observed in the modern instrumental record.

Description: Very few age controls exist for Quaternary deposits over the vast territory of the East Russian Arctic, which hampers dating of major environmental changes in this area and prevents their correlation to climatic changes in the Arctic and Pacific marine domains. We report a newly identified ~177 ka old Rauchua tephra, which has been dispersed over an area of 〉1,500,000 km2 and directly links terrestrial paleoenvironmental archives from Arctic Siberia with marine cores in the northwest Pacific, thus permitting their synchronization and dating. The Rauchua tephra can help to identify deposits formed in terrestrial and marine environments during the oxygen isotope stage 6.5 warming event. Chemical composition of volcanic glass from the Rauchua tephra points to its island-arc origin, while its spatial distribution singles out the Kamchatka volcanic arc as a source. The Rauchua tephra represents a previously unknown, large (magnitude 〉6.5) explosive eruption from the Kamchatka volcanic arc.

Description: Surface ocean iron (Fe) fertilization can affect the marine primary productivity (MPP), thereby impacting on CO2 exchanges at the atmosphere-ocean interface and eventually on climate. Mineral (aeolian or desert) dust is known to be a major atmospheric source for the surface ocean biogeochemical iron cycle, but the significance of volcanic ash is poorly constrained. We present the results of geochemical experiments aimed at determining the rapid release of Fe upon contact of pristine volcanic ash with seawater, mimicking their dry deposition into the surface ocean. Our data show that volcanic ash from both subduction zone and hot spot volcanoes (n = 44 samples) rapidly mobilized significant amounts of soluble Fe into seawater (35–340 nmol/g ash), with a suggested global mean of 200 ± 50 nmol Fe/g ash. These values are comparable to the range for desert dust in experiments at seawater pH (10–125 nmol Fe/g dust) presented in the literature (Guieu et al., 1996; Spokes et al., 1996). Combining our new Fe release data with the calculated ash flux from a selected major eruption into the ocean as a case study demonstrates that single volcanic eruptions have the potential to significantly increase the surface ocean Fe concentration within an ash fallout area. We also constrain the long-term (millennial-scale) airborne volcanic ash and mineral dust Fe flux into the Pacific Ocean by merging the Fe release data with geological flux estimates. These show that the input of volcanic ash into the Pacific Ocean (128–221 × 1015 g/ka) is within the same order of magnitude as the mineral dust input (39–519 × 1015 g/ka) (Mahowald et al., 2005). From the similarity in both Fe release and particle flux follows that the flux of soluble Fe related to the dry deposition of volcanic ash (3–75 × 109 mol/ka) is comparable to that of mineral dust (1–65 × 109 mol/ka). Our study therefore suggests that airborne volcanic ash is an important but hitherto underestimated atmospheric source for the Pacific surface ocean biogeochemical iron cycle.

Description: To understand the gradual global cooling during the mid-Pliocene (3.5–2.5 Myr ago) one needs to consider the tectonical constriction of tropical seaways, which affected ocean circulation and the evolution of climate. Here we use paired measurements of δ18O and Mg/Ca ratios of planktonic foraminifera to reconstruct the Pliocene hydrography of the western tropical Indian Ocean (Site 709C) and changes in the Leeuwin Current in the eastern subtropical Indian Ocean (Site 763A) in response to Indonesian Gateway dynamics. Today, the Indonesian Throughflow (ITF) and, subsequently, the warm southward flowing Leeuwin Current off Western Australia are essential for the polar heat transport in the Indian Ocean. During 3.5–3 Ma, sea surface temperatures significantly dropped in the Leeuwin Current area, becoming since ~3.3 Ma 2°C–3°C cooler than the rather unchanged sea surface temperatures from the eastern and western tropical Indian Ocean. We refer this drop in sea surface temperatures to a weakened Leeuwin Current with severe climatic effects on Western Australia induced by a tectonically reduced surface ITF. We suggest that this reduced surface ITF led to a diminished poleward heat transport in the Indian Ocean resulting in a weakened Leeuwin Current and possibly to cooling of the Benguela upwelling system.

Description: Modeling and proxy studies indicate that a reduction of Atlantic Meridional Overturning Circulation (AMOC) strength profoundly impacts temperatures and salinities in the (sub)tropical Atlantic, especially on subsurface levels. While previous studies focused on prominent periods of AMOC reduction during the last deglaciation, we aim to test whether similar reconfigurations of the subtropical hydrography occurred during the moderate climatic alterations punctuating the last interglacial Marine Isotope Stage (MIS) 5. Here, we present temperature and salinity records from a Florida Straits core by combining d18O and Mg/Ca analyses on surface (Globigerinoides ruber, white) and deep‐dwelling (Globorotalia crassaformis) foraminifera covering MIS 5 in high resolution. The data reveal increasing salinities at intermediate depths during interglacial cooling episodes, decoupled from relatively stable surface conditions. This probably indicates the spatial expansion of saline subtropical gyre waters due to enhanced Ekman downwelling and might also point to a changed density structure and altered geostrophic balance in Florida Straits. Notably, these oceanographic alterations are not consistently occurring during periods of AMOC reduction. The data suggest that the expansion of gyre waters into Florida Straits was impeded by the increasing influence of Antarctic Intermediate Water (AAIW) from MIS 5.5 to ∼107 kyr BP. Afterward, increasingly positive benthic d13C values imply a recession of AAIW, allowing the temporary expansion of gyre waters into Florida Straits. We argue that the inferred transient subtropical salt accumulation and warm pool expansion might have played a pivotal role in reinvigorating meridional overturning and dampen the severity of interglacial cold phases.

Description: Serpentinized peridotite and gabbronorite represent the host rocks to the active, ultramafic-hosted Logatchev hydrothermal field at the Mid-Atlantic Ridge. We use trace element, δ18O and 87Sr/86Sr data from bulk rock samples and mineral separates in order to constrain the controls on the geochemical budget within the Logatchev hydrothermal system. The trace element data of serpentinized peridotite show strong compositional variations indicating a range of processes. Some peridotites experienced geochemical modifications associated with melt-rock interaction processes prior to serpentinization, which resulted in positive correlations of increasing high field strength element (HFSE) concentrations and light rare earth element (LREE) contents. Other serpentinites and lizardite mineral separates are enriched in LREE, lacking a correlation with HFSE due to interaction with high-temperature, black-smoker type fluids. The enrichment of serpentinites and lizardite separates in trace elements, as well as locally developed negative Ce-anomalies, indicate that interaction with low-T ambient seawater is another important process in the Logatchev hydrothermal system. Hence, mixing of high-T hydrothermal fluids during serpentinization and/or re-equilibration of O-isotope signatures during subsequent low-T alteration is required to explain the trace element and δ18O temperature constraints. Highly radiogenic 87Sr/86Sr signatures of serpentinite and lizardite separates provide additional evidence for interaction with seawater-derived fluids. Sparse talc alteration at the Logatchev site are most likely caused by Si-metasomatism of serpentinite associated with the emplacement of shallow gabbro intrusion(s) generating localized hydrothermal circulation. In summary the geochemistry of serpentinites from the Logatchev site document subsurface processes and the evolution of a seafloor ultramafic hydrothermal system.