Precession-driven changes in Iceland–Scotland Overflow Water penetration and bottom water circulation on Gardar Drift since ~ 200 ka

© The Author(s), 2015. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Palaeogeography, Palaeoclimatology, Palaeoecology 440 (2015): 561-563, doi:10.1016/j.palaeo.2015.09.042.Benthic foraminiferal stable isotopic records from a transect of sediment cores south of the Iceland-Scotland Ridge reveal that the penetration depth of Iceland-Scotland Overflow Water (ISOW) varied on orbital timescales with precessional pacing over the past ~ 200 kyr. Similar, higher benthic foraminiferal δ13 C values (~ 1.0 ‰) were recorded at all transect sites downstream of the Iceland-Scotland Ridge during interglacial periods (Marine Isotope Chrons 5 and 1), indicating a deeply penetrating ISOW. During glacial periods (Marine Isotope Chrons 6, 4, and 2), benthic foraminiferal δ13C values from the deeper (2700-3300 m), southern sites within this transect were significantly lower (~ 0.5 ‰) than values from the northern (shallower) portion of the transect (~ 1.0 ‰), reflecting a shoaling of ISOW and greater influence of glacial Southern Component Water (SCW) in the deep Northeast Atlantic. Particularly during intermediate climate states, ISOW strength is driven by precesional cycles, superimposed on the large-scale glacial-interglacial ISOW variability. Millennial-scale variability in the penetration of ISOW, likely caused by high-frequency Heinrich and Dansgaard-Oeschger Events, is most pronounced during intermediate climate states.This research was supported by National Science Foundation grant OCE-0095219 to J.D. Wright2016-10-0

Southwest Pacific subtropics responded to last deglacial warming with changes in shallow water sources

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 29 (2014): 595–611, doi:10.1002/2013PA002584.This study examined sources of mixed layer and shallow subsurface waters in the subtropical Bay of Plenty, New Zealand, across the last deglaciation (~30–5 ka). δ18O and δ13C from planktonic foraminifera Globgerinoides bulloides and Globorotalia inflata in four sediment cores were used to reconstruct surface mixed layer thickness, δ18O of seawater (δ18OSW) and differentiate between high- and low-latitude water provenance. During the last glaciation, depleted planktonic δ18OSW and enriched δ13C (−0.4–0.1‰) indicate surface waters had Southern Ocean sources. A rapid δ13C depletion of ~1‰ in G. bulloides between 20 and 19 ka indicates an early, permanent shift in source to a more distal tropical component, likely with an equatorial Pacific contribution that persisted into the Holocene. At 18 ka, a smaller but similar shift in G. inflata δ13C depletion of ~0.3‰ suggests that deeper subsurface waters had a delayed reaction to changing conditions during the deglaciation. This contrasts with the isotopic records from nearby Hawke Bay, to the east of the North Island of New Zealand, which exhibited several changes in thermocline depth indicating switches between distal subtropical and proximal subantarctic influences during the early deglaciation ending only after the Antarctic Cold Reversal. Our results identify the midlatitude subtropics, such as the area around the North Island of New Zealand, as a key region to decipher high- versus low-latitude influences in Southern Hemisphere shallow water masses.Funding for this project came from NSF OCE-0823487 and 0823549-03.2014-12-1

The distribution and coordination of trace elements in Krithe ostracods and their implications for paleothermometry

The Mg and Sr content of ostracod valves have been used to reconstruct past temperature and salinity, and their stable isotopes have been used to reveal aspects of marine, lake and estuary hydrology. However, significant uncertainties surround ostracod calcification processes, the incorporation mechanisms of trace elements, and the sensitivity of proxy tracers to complex confounding factors. The valves of most ostracods are composed of micron-scale crystalline grains embedded in an organic matrix. The fine-scale geochemistry of these structures, and the nature of the influence of biological mineralisation processes on valve chemistry, remain poorly constrained. We have performed sub-micron resolution X-ray microscopy of a marine Krithe ostracod valve, and determined the chemical coordination of Mg, and the distribution of Mg, Na and S throughout the crystal-organic valve structure. These trace elements display systematic sub-micron-scale compositional variations within the mineral grains and inter-granular matrix of the valve ultrastructure. These patterns imply that Krithe biomineralisation processes significantly modulate trace element incorporation at the sub-micron scale. Thus Krithe chemical composition is likely to be decoupled to some extent from the water in which they calcified. Most importantly, Mg K-edge Near-Edge X-Ray Absorption Fine Structure (NEXAFS) spectra, and the coincidence of high-Mg regions with S-rich organic layers reveal that Mg is not primarily hosted in the calcite structure in the valve. Our results highlight the need to understand the processes that drive this fine-scale chemical heterogeneity and their influence on connections between the external environment and valve geochemistry, if ostracods are to be used as sources of paleoenvironmental proxies.The work was funded by a beamtime proposal to the Bessy II synchrotron, NERC PhD studentships awarded to OB and ER, and by an ERC (2010-NEWLOG ADG-267931 grant to HE)

Six decades of glacier mass changes around Mt. Everest are revealed by historical and contemporary images

The accurate quantification of current and past Himalayan glacier mass budgets is vital if we are to understand the evolution of the Asian water tower, which provides water to the planet’s most populous region. In this work, we generated a geodetic time series spanning six decades over 79 glaciers surrounding Mt. Everest and found consistent acceleration of glacier mass loss between the 1960s (−0.23 ± 0.12 mwe a−1) and the modern era (−0.38 ± 0.11 mwe a−1 from 2009 to 2018). Glacier mass loss has varied depending on glacier terminus type and surface characteristics, and glacier thinning is now occurring at extreme altitudes (>6,000 masl). Our time series also captures the first documented surge of a glacier in the Mt. Everest region. These multi-decadal observations of glacier mass loss form a baseline dataset against which physically based glacier evolution models could be calibrated before they are used for predicting future meltwater yield.Publisher PDFPeer reviewe

Antarctic Intermediate Water properties since 400 ka recorded in infaunal (Uvigerina peregrina) and epifaunal (Planulina wuellerstorfi) benthic foraminifera

Reconstruction of intermediate water properties is important for understanding feedbacks within the ocean-climate system, particularly since these water masses are capable of driving high–low latitude teleconnections. Nevertheless, information about intermediate water mass evolution through the late Pleistocene remains limited. This paper examines changes in Antarctic Intermediate Water (AAIW), the most extensive intermediate water mass in the modern ocean through the last 400 kyr using the stable isotopic composition (δ18O and δ13C) and trace element concentration (Mg/Ca and B/Ca) of two benthic foraminiferal species from the same samples: epifaunal Planulina wuellerstorfi and infaunal Uvigerina peregrina. Our results confirm that the most reasonable estimates of AAIW temperature and Δ[CO2−3] are generated by Mg/CaU. peregrina and B/CaP. wuellerstorfi, respectively. We present a 400 kyr record of intermediate water temperature and Δ[CO2−3] from a sediment core from the Southwest Pacific (DSDP site 593; 40°30′S, 167°41′E, 1068 m water depth), which lies within the core of modern AAIW. Our results suggest that a combination of geochemical analyses on both infaunal and epifaunal benthic foraminiferal species yields important information about this critical water mass through the late Pleistocene. When combined with two nearby records of water properties from deeper depths, our data demonstrate that during interglacial stages of the late Pleistocene, AAIW and Circumpolar Deep Water (CPDW) have more similar water mass properties (temperature and δ13C), while glacial stages are typified by dissimilar properties between AAIW and CPDW in the Southwest Pacific. Our new Δ[CO2−3] record shows short time-scale variations, but a lack of coherent glacial–interglacial variability indicating that large quantities of carbon were not stored in intermediate waters during recent glacial periods

A case study using 2019 pre-monsoon snow and stream chemistry in the Khumbu region, Nepal

This case study provides a framework for future monitoring and evidence for human source pollution in the Khumbu region, Nepal. We analyzed the chemical composition (major ions, major/trace elements, black carbon, and stable water isotopes) of pre-monsoon stream water (4300–5250 m) and snow (5200–6665 m) samples collected from Mt. Everest, Mt. Lobuche, and the Imja Valley during the 2019 pre-monsoon season, in addition to a shallow ice core recovered from the Khumbu Glacier (5300 m). In agreement with previous work, pre-monsoon aerosol deposition is dominated by dust originating from western sources and less frequently by transport from southerly air mass sources as demonstrated by evidence of one of the strongest recorded pre-monsoon events emanating from the Bay of Bengal, Cyclone Fani. Elevated concentrations of human-sourced metals (e.g., Pb, Bi, As) are found in surface snow and stream chemistry collected in the Khumbu region. As the most comprehensive case study of environmental chemistry in the Khumbu region, this research offers sufficient evidence for increased monitoring in this watershed and surrounding areas

Broadening inclusivity at sea

Ocean sciences in the U.S. remains a field with one of the lowest rates of diversity, having disproportionately low representation from marginalized groups, including Black, Asian, LatinX, Indigenous, and other people of color; LGBTQIA+ individuals; disabled persons; women; those with neurological differences; and those from low-income groups. With equity and inclusion in mind, recent efforts have been made to increase the number of ocean science professionals from marginalized groups through multiple entry points, including internships. However, there still exists a large gap between the diversity found in the general population and the diversity within ocean sciences. Perhaps one reason why this field continues to have lower diversity owes to the unique component of many oceanographic careers, which continues to present an especially high barrier for marginalized groups: participating in sea-going research expeditions. Herein, we have synthesized possible ways to prioritize the physical and emotional safety of marginalized ocean science professionals participating in a research expedition, including guidance on preparation, implementation, and providing support post-cruise. These suggestions are intended to be useful for the broader oceanographic research community to consider the safety and well-being of individuals from marginalized groups at sea, since the field of ocean sciences - like all fields - would greatly benefit from increased representation and diversity

Determination of B/Ca of natural carbonates by HR-ICP-MS

We report a new method for HR-ICP-MS based accurate and precise B/Ca determination from low mass natural carbonates (=65 µg CaCO3 to achieve a comparable external precision of 3.5% (2σ). We report a B/Ca detection limit of 2 µmol/mol compared to >=10 µmol/mol for previous methods, a fivefold improvement. The method presented here can determine a wide range of B/Ca (9.0–250 µmol/mol) in mass limited samples with considerable tolerance for matrix matching efficiency (<=±30%). The long-term reproducibility of B/Ca measured on Cambridge in-house consistency standards containing \textless20, \~85, and \~200 µmol/mol of B/Ca are ±3.7% (2σ, n = 100), ±3.9% (2σ, n = 150), and ±3.2% (2 s, n =180), respectively. A host of other trace element to Ca ratios can also be determined at comparable external precision from samples containing <=5 µg CaCO3. This method is suitable for trace element analysis of single foraminifera shel

Mid Pleistocene foraminiferal mass extinction coupled with phytoplankton evolution

Understanding the interaction between climate and biotic evolution is crucial for deciphering the sensitivity of life. An enigmatic mass extinction occurred in the deep oceans during the Mid Pleistocene, with a loss of over 100 species (20%) of sea floor calcareous foraminifera. An evolutionarily conservative group, benthic foraminifera often comprise >50% of eukaryote biomass on the deep-ocean floor. Here we test extinction hypotheses (temperature, corrosiveness and productivity) in the Tasman Sea, using geochemistry and micropalaeontology, and find evidence from several globally distributed sites that the extinction was caused by a change in phytoplankton food source. Coccolithophore evolution may have enhanced the seasonal ‘bloom’ nature of primary productivity and fundamentally shifted it towards a more intra-annually variable state at ∼0.8 Ma. Our results highlight intra-annual variability as a potential new consideration for Mid Pleistocene global biogeochemical climate models, and imply that deep-sea biota may be sensitive to future changes in productivity

Pliocene-Pleistocene evolution of sea surface and intermediate water temperatures from the southwest Pacific

Over the last 5 million years, the global climate system has evolved toward a colder mean state, marked by large-amplitude oscillations in continental ice volume. Equatorward expansion of polar waters and strengthening temperature gradients have been detected. However, the response of the mid latitudes and high latitudes of the Southern Hemisphere is not well documented, despite the potential importance for climate feedbacks including sea ice distribution and low-high latitude heat transport. Here we reconstruct the Pliocene-Pleistocene history of both sea surface and Antarctic Intermediate Water (AAIW) temperatures on orbital time scales from Deep Sea Drilling Project Site 593 in the Tasman Sea, southwest Pacific. We confirm overall Pliocene-Pleistocene cooling trends in both the surface ocean and AAIW, although the patterns are complex. The Pliocene is warmer than modern, but our data suggest an equatorward displacement of the subtropical front relative to present and a poleward displacement of the subantarctic front of the Antarctic Circumpolar Current (ACC). Two main intervals of cooling, from ~3 Ma and ~1.5 Ma, are coeval with cooling and ice sheet expansion noted elsewhere and suggest that equatorward expansion of polar water masses also characterized the southwest Pacific through the Pliocene-Pleistocene. However, the observed trends in sea surface temperature and AAIW temperature are not identical despite an underlying link to the ACC, and intervals of unusual surface ocean warmth (~2 Ma) and large-amplitude variability in AAIW temperatures (from ~1 Ma) highlight complex interactions between equatorward displacements of fronts associated with the ACC and/or varying poleward heat transport from the subtropics