ALBERT

Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 31 (2016): 626–638, doi:10.1002/2015PA002908.

Description: Coral skeletons archive past climate variability with unrivaled temporal resolution. However, extraction of accurate temperature information from coral skeletons has been limited by “vital effects,” which confound, and sometimes override, the temperature dependence of geochemical proxies. We present a new approach to coral paleothermometry based on results of abiogenic precipitation experiments interpreted within a framework provided by a quantitative model of the coral biomineralization process. DeCarlo et al. (2015a) investigated temperature and carbonate chemistry controls on abiogenic partitioning of Sr/Ca and U/Ca between aragonite and seawater and modeled the sensitivity of skeletal composition to processes occurring at the site of calcification. The model predicts that temperature can be accurately reconstructed from coral skeleton by combining Sr/Ca and U/Ca ratios into a new proxy, which we refer to hereafter as the Sr-U thermometer. Here we test the model predictions with measured Sr/Ca and U/Ca ratios of 14 Porites sp. corals collected from the tropical Pacific Ocean and the Red Sea, with a subset also analyzed using the boron isotope (δ11B) pH proxy. Observed relationships among Sr/Ca, U/Ca, and δ11B agree with model predictions, indicating that the model accounts for the key features of the coral biomineralization process. By calibrating to instrumental temperature records, we show that Sr-U captures 93% of mean annual temperature variability (26–30°C) and has a standard deviation of prediction of 0.5°C, compared to 1°C using Sr/Ca alone. The Sr-U thermometer may offer significantly improved reliability for reconstructing past ocean temperatures from coral skeletons.

Description: NSF Grant Numbers: OCE-1338320, OCE-1031971, OCE-1220529; NSF Graduate Research Fellowships

Description: 2016-12-11

Description: The data consist of ~600 U-Th ages of scleractianian cold-water corals dated by laser ablation and isotope dilution methods covering the last 150,000 years. The corals are from three locations: Reykjanes Ridge (57°N to 61°N, 28°W to 33°W); Tropic Seamount (23°55'N, 20°45'W); and the East Equatorial Atlantic from Carter (9°N, 21°W) and Knipovich seamounts (5°N, 27°W). The samples were collected with ROV and dredges during the cruises: CE0806 in 2008 (Reykjanes Ridge); JC094 in 2013 (Equatorial Atlantic); and JC142 in 2016 (Tropic Seamount). Additionally, a compilation of ~750 U-Th and 14C ages of scleractianian cold-water corals from the Northeast Atlantic Ocean is presented. The complete dataset is used to investigate the temporal and spatial coral distribution at Northeast Atlantic Ocean and the relation with past climatic events.

Description: U-series ages of deep-sea corals from east Equatorial Atlantic and Reykjanes Ridge defined by laser ablation method. Depths of east Equatorial Atlantic corals was retrieved by ROV. Depths of Reykjanes Ridge corals corresponds to the mean depth of the dredge on and off bottom, and depth range is the depth interval between the dredge on and off bottom. Selection code column indicates samples included (accepted) and not included (rejected) on age distribution discussion.

Description: Uranium series dated cold-water corals from Tropic Seamount, Reykjanes Ridge, and East Equatorial Atlantic. Ages are reported in years before present (BP; where present is the year of 1950 CE) both uncorrected and corrected for initial 232Th. Some samples did not pass our quality control. Selection code column indicates samples not included on age distribution discussion: [238U] 〈2 ppm (Low U), [232Th] 〉6 ppb (High Th), [δ234Ui] 〉157‰ (High δU) and lowest quality sub-sample (duplicate).

Description: To better understand the links between the carbon cycle and changes in past climate over tectonic timescales we need new geochemical proxy records of secular change in silicate weathering rates. A number of proxies are under development, but some of the most promising (e.g. palaeoseawater records of Li and Nd isotope change) can only be employed on such large samples of mono-specific foraminifera that application to the deep sea sediment core archive becomes highly problematic. "Dentoglobigerina" venezuelana presents a potentially attractive target for circumventing this problem because it is a typically large (〉 355 mm diameter), abundant and cosmopolitan planktic foraminifer that ranges from the early Oligocene to early Pliocene. Yet considerable taxonomic and ecological uncertainties associated with this taxon must first be addressed. Here, we assess the taxonomy, palaeoecology, and ontogeny of "D." venezuelana using stable isotope (oxygen and carbon) and Mg/Ca data measured in tests of late Oligocene to early Miocene age from Ocean Drilling Program (ODP) Site 925, on Ceara Rise, in the western equatorial Atlantic. To help constrain the depth habitat of "D." venezuelana relative to other species we report the stable isotope composition of selected planktic foraminifera species within Globigerina, Globigerinoides, Paragloborotalia and Catapsydrax. We define three morphotypes of "D." venezuelana based on the morphology of the final chamber and aperture architecture. We determine the trace element and stable isotope composition of each morphotype for different size fractions, to test the validity of pooling these morphotypes for the purposes of generating geochemical proxy datasets and to assess any ontogenetic variations in depth habitat. Our data indicate that "D." venezuelana maintains a lower thermocline depth habitat at Ceara Rise between 24 and 21 Ma. Comparing our results to published datasets we conclude that this lower thermocline depth ecology for the Oligo-Miocene is part of an Eocene-to-Pliocene evolution of depth habitat from surface to sub-thermocline for "D." venezuelana. Our size fraction data advocate the absence of photosymbionts in "D." venezuelana and suggest that juveniles calcify higher in the water column, descending into slightly deeper water during the later stages of its life cycle. Our morphotype data show that d18O and d13C variation between morphotypes is no greater than within-morphotype variability. This finding will permit future pooling of morphotypes in the generation of the "sample hungry" palaeoceanographic records.

Description: Compilation of cold-water coral dated by U-series or radiocarbon of Northeast Atlantic Ocean. Age column corresponds to reported U-series corrected ages or calendar 14C ages re-calculated. Re-calibrated 14C ages column corresponds to 14C ages calculated using CALIB8.10 software, Marine20 calibration curve, and age offset with Marine20 indicated at column "Local offset with Marine20".

Description: The Antarctic Cold Reversal (ACR; 14.7 to 13 ka) phase of the last deglaciation saw a pause in the rise of atmospheric pCO2 and Antarctic temperature, contrasted with warming in the North. Mechanisms associated with interhemispheric heat transfer have been proposed to explain features of this event, but the response of marine biota and the carbon cycle are debated. The Southern Ocean is a key site of deep-water exchange with the atmosphere, hence deglacial changes in nutrient cycling, circulation, and productivity in this region may have global impact. Here we present a new perspective on the sequence of events in the deglacial Southern Ocean, that includes multi-faunal benthic assemblage (foraminifera and cold-water corals) and geochemical data (Ba/Ca, 14C, δ11B) from the Drake Passage. Our records feature anomalies during peak ACR conditions indicative of circulation, biogeochemistry, and regional ecosystem perturbations. Within this cold episode, peak abundances of thick-walled benthic foraminifera and cold-water corals are observed at shallow depths in the sub-Antarctic (~300 m), while coral populations at greater depths and further south diminished. Geochemical data indicate that habitat shifts were associated with enhanced primary productivity in the sub-Antarctic, a more stratified water column, and poorly oxygenated bottom water. These results are consistent with northward migration of primary production in response to Antarctic cooling and widespread biotic turnover across the Southern Ocean. We suggest that expanding sea ice, suppressed ventilation, and shifting centres of upwelling drove changes in planktic and benthic ecology, and were collectively instrumental in halting CO2 rise in the mid-deglaciation.

Description: Fossil scleractinian corals were collected from the Galápagos platform in the East Equatorial Pacific (0°N, 90°E) on cruises MV1007 and NA064 from water depths between 419 and 650 m. Equatorial Atlantic corals (taxa Caryophyllia, Enallopsammia, Desmophyllum) were collected from a depth range of 749 to 2814 m during Cruise JC094 from Carter Seamount (9.2°N, 21.3°W), Knipovich Seamount (5.6°N, 26.9°W), Vema Fracture Zone (10.7°N, 44.6°W), Vayda Seamount (14.9°N, 48.2°W) and Gramberg Seamount (15.4°N, 51.1°W). Southern Ocean samples were obtained from Burdwood Bank (54.7°S, 62.2°W; taxa Caryophyllia, Balanophyllia, Flabellum, Desmophyllum) and Cape Horn (57.2°S, 67.1°W; taxa Caryophyllia, Balanophyllia, Flabellum) in the Subantarctic Zone and the Sars and Interim Seamounts in the Polar Front Zone (59.7°S, 68.8°W and 60.6°S, 66.0°W; taxa Caryophyllia, Desmophyllum) on cruises NBP0805 and NBP1103 in the Drake Passage. These proximal Sars and Interim sites are grouped as simply "Sars". The shallowest coral samples come from depths of 334 m on Burdwood Bank however the majority are from 700 to 1520 m, at water depths corresponding to modern Antarctic Intermediate Water. Corals recovered from the depth of 1012 m from Cape Horn and further south from Sars Seamount at depths of 695 to 1200 m are currently bathed in Upper Circumpolar Deep Water. Deeper samples at the Sars Seamount site sit within Lower Circumpolar Deep Water (1300 to 1750 m). We use published U-series dates for all samples (Burke and Robinson, 2012; Chen et al., 2020; Chen et al., 2015; Li et al., 2020; Margolin et al., 2014; Stewart et al., 2021). Reported age uncertainties are typically ±1% (2 SD). Whole "S1" septa and attached theca were taken from cup corals while whole calyxes were taken from branching specimens using a rotary cutting tool. This tool was further used to remove surficial oxide coatings and any chalky altered carbonate. Where sufficient sample material allowed, multiple sub-samples were measured to minimize microstructural bias (typically duplicates). Coral fragments were crushed and cleaned using warm 1% H2O2 (buffered in NH4OH) oxidative cleaning and a weak acid polish (0.0005 M HNO3). Samples were dissolved in 0.5 M HNO3 and analysed by ICP-MS to yield Li/Mg ratios. Repeat analysis of NIST RM 8301 (Coral) (n=19) yielded analytical precision of 〈± 1.5%. Coral Li/Mg was converted to temperature using a calibration applicable to all aragonitic corals (Li/Mg = 5.42 exp(−0.050×T(°C)); (Stewart et al., 2020). The quoted uncertainty on this calibration based on prediction intervals is ± 1.7 °C (1σ). This uncertainty is significantly reduced however at extremely low temperatures close to the freezing point of seawater (~ −2 °C). Corals could not survive in frozen seawater, therefore, where proxy estimated temperature falls below this minimum a value of −2 °C is reported instead. For Li/Mg averages of each coral sample and conversion to bottom water temperature, see the xlsx version of the dataset under Further details.

Description: The Antarctic Cold Reversal (ACR; 14.7 to 13 ka) phase of the last deglaciation saw a pause in the rise of atmospheric pCO2 and Antarctic temperature, contrasted with warming in the North. Mechanisms associated with interhemispheric heat transfer have been proposed to explain features of this event, but the response of marine biota and the carbon cycle are debated. The Southern Ocean is a key site of deep-water exchange with the atmosphere, hence deglacial changes in nutrient cycling, circulation, and productivity in this region may have global impact. Here we present a new perspective on the sequence of events in the deglacial Southern Ocean, that includes multi-faunal benthic assemblage (foraminifera and cold-water corals) and geochemical data (Ba/Ca, 14C, δ11B) from the Drake Passage. Our records feature anomalies during peak ACR conditions indicative of circulation, biogeochemistry, and regional ecosystem perturbations. Within this cold episode, peak abundances of thick-walled benthic foraminifera and cold-water corals are observed at shallow depths in the sub-Antarctic (~300 m), while coral populations at greater depths and further south diminished. Geochemical data indicate that habitat shifts were associated with enhanced primary productivity in the sub-Antarctic, a more stratified water column, and poorly oxygenated bottom water. These results are consistent with northward migration of primary production in response to Antarctic cooling and widespread biotic turnover across the Southern Ocean. We suggest that expanding sea ice, suppressed ventilation, and shifting centres of upwelling drove changes in planktic and benthic ecology, and were collectively instrumental in halting CO2 rise in the mid-deglaciation.

Description: The Antarctic Cold Reversal (ACR; 14.7 to 13 ka) phase of the last deglaciation saw a pause in the rise of atmospheric pCO2 and Antarctic temperature, contrasted with warming in the North. Mechanisms associated with interhemispheric heat transfer have been proposed to explain features of this event, but the response of marine biota and the carbon cycle are debated. The Southern Ocean is a key site of deep-water exchange with the atmosphere, hence deglacial changes in nutrient cycling, circulation, and productivity in this region may have global impact. Here we present a new perspective on the sequence of events in the deglacial Southern Ocean, that includes multi-faunal benthic assemblage (foraminifera and cold-water corals) and geochemical data (Ba/Ca, 14C, δ11B) from the Drake Passage. Our records feature anomalies during peak ACR conditions indicative of circulation, biogeochemistry, and regional ecosystem perturbations. Within this cold episode, peak abundances of thick-walled benthic foraminifera and cold-water corals are observed at shallow depths in the sub-Antarctic (~300 m), while coral populations at greater depths and further south diminished. Geochemical data indicate that habitat shifts were associated with enhanced primary productivity in the sub-Antarctic, a more stratified water column, and poorly oxygenated bottom water. These results are consistent with northward migration of primary production in response to Antarctic cooling and widespread biotic turnover across the Southern Ocean. We suggest that expanding sea ice, suppressed ventilation, and shifting centres of upwelling drove changes in planktic and benthic ecology, and were collectively instrumental in halting CO2 rise in the mid-deglaciation.