ALBERT

Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Paleoceanography and Paleoclimatology 33 (2018): 1013-1034, doi:10.1029/2018PA003408.

Description: The chemical composition of benthic foraminifera from marine sediment cores provides information on how glacial subsurface water properties differed from modern, but separating the influence of changes in the origin and end‐member properties of subsurface water from changes in flows and mixing is challenging. Spatial gaps in coverage of glacial data add to the uncertainty. Here we present new data from cores collected from the Demerara Rise in the western tropical North Atlantic, including cores from the modern tropical phosphate maximum at Antarctic Intermediate Water (AAIW) depths. The results suggest lower phosphate concentration and higher carbonate saturation state within the phosphate maximum than modern despite similar carbon isotope values, consistent with less accumulation of respired nutrients and carbon, and reduced air‐sea gas exchange in source waters to the region. An inversion of new and published glacial data confirms these inferences and further suggests that lower preformed nutrients in AAIW, and partial replacement of this still relatively high‐nutrient AAIW with nutrient‐depleted, carbonate‐rich waters sourced from the region of the modern‐day northern subtropics, also contributed to the observed changes. The results suggest that glacial preformed and remineralized phosphate were lower throughout the upper Atlantic, but deep phosphate concentration was higher. The inversion, which relies on the fidelity of the paleoceanographic data, suggests that the partial replacement of North Atlantic sourced deep water by Southern Ocean Water was largely responsible for the apparent deep North Atlantic phosphate increase, rather than greater remineralization.

Description: National Science Foundation (NSF) Grant Numbers: OCE‐0750880, OCE‐1335191, OCE‐1558341, OCE‐1536380; Woods Hole Oceanographic Institution (WHOI) Grant Numbers: 27007592, 27000808

Description: During the past decades Mg/Ca ratios have been increasingly used in order to calculate past temperature variations independent from faunal assemblages. Especially in the Fram Strait, the main pathway of heat flux to the Arctic, new temperature estimation tools are urgently needed to better understand past complex interaction of different water masses and the extent of Atlantic Water advection to the Arctic Ocean. The Holocene section of a sediment core from the western Svalbard margin has been studied at high-resolution for benthic proxy indicators to reconstruct deepwater sources and mixing in the Arctic Gateway since the last ca 10,000 years. Benthic stable isotope values and sortable silt mean grain size data are compared to a first, preliminary data set of Mg/Ca paleotemperatures established from the benthic foraminifer species Cibicidoides wuellerstorfi in the eastern Fram Strait. When compared to planktic proxy indicators, this reconstruction of past bottom water temperatures at a northernmost site allows to estimate the linkage between deepwater inflow and AW advection within the West Spitsbergen Current. Furthermore, benthic Mg/Ca temperatures can help unravelling the local impact (e.g., by brine-enriched waters) from general trends in bottom water circulation. Short-lived decreases in benthic carbon isotope values seem to correlate to cold surface water events in the area such as the 8.2 ka event. Similarly, decreases in benthic carbon isotope values in the Nordic Seas around 8 ka have been assigned to decreased bottom water ventilation possibly due to an entrainment of relatively fresh water into the thermohaline system (Bauch et al., 2001). While sluggish bottom current speeds have been found for the 8.2 ka event north of our site on the Yermak Plateau (Hass, 2002), during colder events on the Western Svalbard margin sediment data seem to anticorrelate to benthic carbon isotope data either suggesting a rather unexpected increase in bottom current velocity or an impact of brine-enriched winter waters from the fjord/trough system which might have generated increased lateral coarser-grained sediment injections (Sarnthein et al., 2003). A Late Holocene trend towards significantly higher benthic oxygen isotopes may be either related to a cooling or increasing salinity in bottom waters. Higher salinity of bottom waters may be again caused by dense water formation during winter sea-ice formation in southern and western Svalbard fjords (e.g., Quadfasel et al., 1988; Rudels et al., 2005). Bauch, H. A., H. Erlenkeuser, R. F. Spielhagen, U. Struck, J. Matthiessen, J. Thiede, and J. Heinemeier (2001a), A multiproxy reconstruction of the evolution of deep and surface waters in the subarctic Nordic seas over the last 30,000 yr, Quaternary Science Reviews, 20(4), 659-678. Hass, H. C. (2002), A method to reduce the influence of ice-rafted debris on a grain size record from northern Fram Strait, Polar Research, 21(2), 299-306. Quadfasel, D., B. Rudels, and K. Kurz (1988), Outflow of dense water from a Svalbard fjord into the Fram Strait, Deep Sea Research Part A. Oceanographic Research Papers, 35(7), 1143-1150. Rudels, B., G. Bjork, J. Nilsson, P. Winsor, I. Lake, and C. Nohr (2005), The interaction between waters from the Arctic Ocean and the Nordic Seas north of Fram Strait and along the East Greenland Current: results from the Arctic Ocean-02 Oden expedition, Journal of Marine Systems, 55(1-2), 1-30. Sarnthein, M., S. van Krefeldt, H. Erlenkeuser, P. M. Grootes, M. Kucera, U. Pflaumann, and M. Schulz (2003), Centennial-to-millennial-scale periodicities of Holocene climate and sediment injections off the western Barents shelf, 75◦N, Boreas, 32, 447-461.

Description: Over the past decade, the ratio of Mg to Ca in foraminiferal tests has emerged as a valuable paleotemperature proxy. However, large uncertainties remain in the relationships between benthic foraminiferal Mg/Ca and temperature. Mg/Ca was measured in benthic foraminifera from 31 high-quality multicore tops collected in the Florida Straits, spanning a temperature range of 5.8° to 18.6°C. New calibrations are presented for Uvigerina peregrina, Planulina ariminensis, Planulina foveolata, and Hoeglundina elegans. The Mg/Ca values and temperature sensitivities vary among species, but all species exhibit a positive correlation that decreases in slope at higher temperatures. The decrease in the sensitivity of Mg/Ca to temperature may potentially be explained by Mg/Ca suppression at high carbonate ion concentrations. It is suggested that a carbonate ion influence on Mg/Ca may be adjusted for by dividing Mg/Ca by Li/Ca. The Mg/Li ratio displays stronger correlations to temperature, with up to 90% of variance explained, than Mg/Ca alone. These new calibrations are tested on several Last Glacial Maximum (LGM) samples from the Florida Straits. LGM temperatures reconstructed from Mg/Ca and Mg/Li are generally more scattered than core top measurements and may be contaminated by high-Mg overgrowths. The potential for Mg/Ca and Mg/Li as temperature proxies warrants further testing.

Description: The dataset contains foraminifera images of over 1,000 forams taken under 16 different lighting directions with an optical microscope. The species and locations of the samples are also specified. It also contains manual segmentation of over 400 samples from the images described above. The segmentation labels are matched by their name. To capture these images, a visual identification system was developed in order to automate the identification of target microorganisms. The visual system incorporates a controllable LED lighting ring used to capture images by illuminating the specimens from several directions, mimicking an important step in the traditional identification process. The dataset was originally used for foraminifera identification and segmentation with machine learning and computer vision techniques. This work is a collaboration between the Dr. Edgar Lobaton (Associate Professor at the North Carolina State University), Dr. Thomas Marchitto (Associate Professor at the University of Colorado Boulder) and Dr. Ritayan Mitra (Assistant Professor at IIT Bombay). Please refer to https://research.ece.ncsu.edu/aros/foram-identification/ for more information about the datasets, related studies and downloading the dataset.

Description: Benthic foraminiferal delta13C suggests that there was a net shift of isotopically light metabolic CO2 from the upper ocean into the deep ocean during the last glacial period. According to the 'CaCO3 compensation' hypothesis, this should have caused a transient drop in deep ocean CO3[2-] that was eventually reversed by seafloor dissolution of CaCO3. The resulting increase in whole-ocean pH may have had a significant impact on atmospheric CO2, compounding any decrease that was due to the initial vertical CO2 shift. The opposite hypothetically occurred during deglaciation, when CO2 was returned to the upper ocean (and atmosphere) and deep ocean CO3[2-] temporarily increased, followed by excess burial of CaCO3 and a drop in whole-ocean pH. The deep sea record of CaCO3 preservation appears to reflect these processes, with the largest excursion during deglaciation (as expected), but various factors make quantification of deep sea paleo-CO3[2-] difficult. Here we reconstruct deep equatorial Pacific CO3[2-] over the last glacial-interglacial cycle using benthic foraminiferal Zn/Ca, which is strongly affected by saturation state during calcite precipitation. Our data are in agreement with the CaCO3 compensation theory, including glacial CO3[2-] concentrations similar to (or slightly lower than) today, and a Termination I CO3[2-] peak of ~25-30 µmol kg**-1. The deglacial CO3[2-] rise precedes ice sheet melting, consistent with the timing of the atmospheric CO2 rise. A later portion of the peak could reflect removal of CO2 from the atmosphere-ocean system due to boreal forest regrowth. CaCO3 compensation alone may explain more than one third of the atmospheric CO2 lowering during glacial times.

Description: The recent development of foraminiferal Mg/Ca as a paleotemperature proxy has enabled the extraction of global ice volume and local salinity from the more traditional paleotemperature proxy d18O. The benthic foraminiferal genus Cibicidoides is widely used in paleoceanographic reconstructions because of its epifaunal habitat and cosmopolitan distribution, and it has received early attention in Mg/Ca work. However, existing temperature calibrations for Cibicidoides rely heavily on C. pachyderma core top data from one location, Little Bahamas Bank, where authigenic processes and/or reworking may result in elevated warm water Mg/Ca values. Here we present new C. pachyderma Mg/Ca data from a series of 29 high-quality multicore tops collected in the Florida Straits, spanning a temperature range of 5.8-18.6°C. In contrast to previous calibrations, we find no evidence for a strongly exponential response to temperature. The data are best explained by a linear relationship, with a sensitivity of 0.12 mmol/mol per °C.