Abstract. Radiocarbon dating marine sediments is complicated by the strongly heterogeneous age of ocean waters. Tephrochronology provides a well established method to constrain the age of local radiocarbon reservoirs and more accurately calibrate dates. Numerous ultra-distal cryptotephra deposits (non-visible volcanic ash >3000 km from source) have been identified in peatlands and lake sediments across north-eastern North America, and correlated with volcanic arcs in the Pacific north-west. Previously, however, these isochrons have never been identified in sediments from the north-west Atlantic Ocean. In this study, we report the presence of two ultra-distal cryptotephra deposits; Mazama Ash and White River Ash eastern lobe (WRAe), in Placentia Bay, North Atlantic Ocean. We use these well dated isochrons to constrain the local marine radiocarbon reservoir offset (ΔR) and develop a robust Bayesian age-depth model with a ΔR that varies through time. Our results indicate that the marine radiocarbon offset in Placentia Bay was -126±151 years (relative to the Marine20 calbration curve) at the time of Mazama Ash deposition (7622±18 C.E.) and -396±144 years at the time of WRAe deposition (852–853 C.E.). Changes in ΔR coincide with inferred shifts in relative influences of the Labrador Current and the Slopewater curret in the bay. An important conclusion is that single-offset models of ΔR are easiest to apply and often hard to disprove. However, such models may oversimplify reservoir effects in a core, even over relatively short time scales. Acknowledging potentially varying offsets is critical when ocean circulation and ventilation characteristics have differed over time. The addition of tephra isochrons permits the calculation of semi-independent reservoir corrections and verification of the single ΔR model.
For decades, the Old Crow tephra has been a prominent stratigraphic marker for the onset of Marine Isotope Stage (MIS) 5e, the last interglaciation, in subarctic northwest North America. However, new zircon U-Pb dates for the tephra suggest that the tephra was deposited ca. 207 ka during MIS 7, with wide-ranging implications for chronologies of glaciation, paleoclimate, relict permafrost, and phylogeography. We analyzed ~1900 detrital glass shards from 28 samples collected at Integrated Ocean Drilling Program Site U1345 in the Bering Sea, which has a well-constrained age model from benthic foraminiferal δ18O. Except for one possibly contaminant shard dated at 165 ka, Old Crow tephra was absent from all samples spanning 220–160 ka. Old Crow tephra appeared abruptly at 157 ka, comprising >40% of detrital shards between 157 and 142 ka. This abrupt increase in the concentration of detrital Old Crow tephra, its absence in earlier intervals, and its presence at low concentrations in all samples between 134 and 15 ka collectively indicate that the tephra was deposited during the middle of MIS 6 with a likely age of 159 ± 8 ka. As a result, the late Quaternary chronostratigraphic framework for unglaciated northwest North America remains intact, and the timing of key events in the region (e.g., bison entry into North America; interglacial paleoclimate; permafrost history; the penultimate glaciation) does not require wholesale revision.
Abstract. Radiocarbon dating marine sediments is complicated by the strongly heterogeneous age of ocean waters. Tephrochronology provides a well-established method to constrain the age of local radiocarbon reservoirs and more accurately calibrate dates. Numerous ultra-distal cryptotephra deposits (non-visible volcanic ash more than 3000 km from source) have been identified in peatlands and lake sediments across north-eastern North America and correlated with volcanic arcs in the Pacific north-west. Previously, however, these isochrons have not been identified in sediments from the north-west Atlantic Ocean. In this study, we report the presence of two ultra-distal cryptotephra deposits; Mazama Ash and White River Ash eastern lobe (WRAe), in Placentia Bay, North Atlantic Ocean. We use these well-dated isochrons to constrain the local marine radiocarbon reservoir offset (ΔR) and develop a robust Bayesian age–depth model with a ΔR that varies through time. Our results indicate that the marine radiocarbon offset in Placentia Bay was -126±151 years (relative to the Marine20 calibration curve) at the time of Mazama Ash deposition (7572 ± 18 yr BP) and −396 ± 144 years at the time of WRAe deposition (1098–1097 yr BP). Changes in ΔR appear to coincide with inferred shifts in relative influences of the inner Labrador Current and the Slopewater Current in the bay. An important conclusion is that single-offset models of ΔR are easiest to apply and often hard to disprove. However, such models may oversimplify reservoir effects in a core, even over relatively short timescales. Acknowledging potentially varying offsets is critical when ocean circulation and ventilation characteristics have differed over time. The addition of tephra isochrons permits the calculation of semi-independent reservoir corrections and verification of the single ΔR model.
Aims: The functional-organic distinction attempts to differentiate disorders with diagnosable biological causes from those without and is a central axis on which diagnoses, medical specialities, and services are organised. Previous studies report poor agreement between clinicians regarding the meanings of the terms and the conditions to which they apply, as well as noting value-laden implications of relevant diagnoses. Consequently, we aimed to understand how clinicians working in psychiatry and neurology services navigate the functional-organic distinction in their work. Method: Twenty clinicians (10 physicians, 10 psychologists) working in psychiatry and neurology services participated in semi-structured interviews that were analysed applying a constructivist grounded theory approach. Results: The distinction was described as often incongruent with how clinicians conceptualise patients' problems. Organic factors were considered to be objective, unambiguously identifiable, and clearly causative, whereas functional causes were invisible and to be hypothesised through thinking and conversation. Contextual factors - including cultural assumptions, service demands, patient needs, and colleagues' views - were key in how the distinction was deployed in practice. The distinction was considered theoretically unsatisfactory, eventually to be superseded, but clinical decision-making required it to be used strategically. These uses included helping communicate medical problems, navigating services, hiding meaning by making psychological explanations more palatable, tackling stigma, giving hope, and giving access to illness identity. Clinicians cited moral issues at both individual and societal levels as integral to the conceptual basis and deployment of the functional-organic distinction and described actively navigating these as part of their work. Conclusions: There is a considerable distance between the status of the functional-organic distinction as a sound theoretical concept generalisable across conditions and its role as a gatekeeping tool within the structures of healthcare. Ambiguity and contradictions were considered as both obstacles and benefits when deployed in practice and strategic considerations were central in deciding which to lean on.
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