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Geology topics

Harry J. Dowsett

Publications and source records attributed to Harry J. Dowsett.

At least 37 records · Page 2Linked to original sources
USGS2022

Using paleoecological data to inform decision making: A deep-time perspective

Latest climate models project conditions for the end of this century that are generally outside of the human experience. These future conditions affect the resilience and sustainability of ecosystems, alter biogeographic zones, and impact biodiversity. Deep-time records of paleoclimate provide insight into the climate system over millions of years and provide examples of conditions very different from the present day, and in some cases similar to model projections for the future. In addition, the deep-time paleoecologic and sedimentologic archives provide insight into how species and habitats responded to past climate conditions. Thus, paleoclimatology provides essential context for the scientific understanding of climate change needed to inform resource management policy decisions. The Pliocene Epoch (5.3–2.6 Ma) is the most recent deep-time interval with relevance to future global warming. Analysis of marine sediments using a combination of paleoecology, biomarkers, and geochemistry indicates a global mean annual temperature for the Late Pliocene (3.6–2.6 Ma) ∼3°C warmer than the preindustrial. However, the inability of state-of-the-art climate models to capture some key regional features of Pliocene warming implies future projections using these same models may not span the full range of plausible future climate conditions. We use the Late Pliocene as one example of a deep-time interval relevant to management of biodiversity and ecosystems in a changing world. Pliocene reconstructed sea surface temperatures are used to drive a marine ecosystem model for the North Atlantic Ocean. Given that boundary conditions for the Late Pliocene are roughly analogous to present day, driving the marine ecosystem model with Late Pliocene paleoenvironmental conditions allows policymakers to consider a future ocean state and associated fisheries impacts independent of climate models, informed directly by paleoclimate information.

Frontiers in Ecology and Evolution↗
USGS2022

Past terrestrial hydroclimate sensitivity controlled by Earth system feedbacks

Despite tectonic conditions and atmospheric CO 2 levels ( pCO 2 ) similar to those of present-day, geological reconstructions from the mid-Pliocene (3.3-3.0 Ma) document high lake levels in the Sahel and mesic conditions in subtropical Eurasia, suggesting drastic reorganizations of subtropical terrestrial hydroclimate during this interval. Here, using a compilation of proxy data and multi-model paleoclimate simulations, we show that the mid-Pliocene hydroclimate state is not driven by direct CO 2 radiative forcing but by a loss of northern high-latitude ice sheets and continental greening. These ice sheet and vegetation changes are long-term Earth system feedbacks to elevated pCO 2 . Further, the moist conditions in the Sahel and subtropical Eurasia during the mid-Pliocene are a product of enhanced tropospheric humidity and a stationary wave response to the surface warming pattern, which varies strongly with land cover changes. These findings highlight the potential for amplified terrestrial hydroclimate responses over long timescales to a sustained CO 2 forcing.

Nature Communications↗
USGS2022

Very high Middle Miocene surface productivity on the U.S. mid-Atlantic shelf amid glacioeustatic sea level variability

The Miocene Climatic Optimum (MCO) provides important insights into how the climate system operates under elevated temperatures and atmospheric CO 2 levels. Few western North Atlantic paleotemperature or paleoecological records exist from the MCO, despite their importance for understanding both regional and global climate dynamics. Here we present quantitative MCO paleoecological data from the western North Atlantic, specifically from the Baltimore Gas & Electric (BG&E) marine sediment core from southern Maryland, USA. We examine alkenones and planktic foraminifera and document the first sea surface temperature (SST) and productivity estimates for the MCO and the Middle Miocene Climate Transition (MMCT) from the continental shelf. Increased levels of planktic foraminifer species diversity and surface productivity accompany high sea level intervals of the MCO, indicating coastal upwelling. Cooling episodes correlate to unconformities in the BG&E core that reflect sea level lowstands; these and sedimentary cycles tie the record to eccentricity-paced Antarctic ice sheet growth and decay. This dynamic record not only captures the variability in SST, sea level and coastal productivity during the warm MCO and the transition to cooler global temperatures during the MMCT, but it also demonstrates the variability in local conditions within and between intervals of high sea level.

Maryland↗
USGS2021

The Yorktown Formation: Improved stratigraphy, chronology and paleoclimate interpretations from the U.S. mid-Atlantic Coastal Plain

The Yorktown Formation records paleoclimate conditions along the mid-Atlantic Coastal Plain during the mid-Piacenzian Warm Period (3.264 to 3.025 Ma), a climate interval of the Pliocene in some ways analogous to near future climate projections. To gain insight into potential near future changes, we investigated Yorktown Formation outcrops and cores in southeastern Virginia, refining the stratigraphic framework. We analyzed 485 samples for alkenone-based sea surface temperature (SST) and productivity estimates from the Holland and Dory cores, an outcrop at Morgarts Beach, Virginia, and the lectostratotype of the Yorktown Formation at Rushmere, Virginia, and analyzed planktonic foraminferal assemblage data from the type section. Using the structure of the SST record, we improved the chronology of the Yorktown Formation by establishing the maximum age ranges of the Rushmere (3.3–3.2 Ma) and Morgarts Beach (3.2–3.15 Ma) Members. SST values for these members average ~26 °C, corroborating existing sclerochronological data. Increasing planktonic foraminifer abundance, productivity, and species diversity parallel increasing SST over the MIS M2/M1 transition. These records constitute the greatest temporal concentration of paleoecological estimates within the Yorktown Formation, aiding our understanding of western North Atlantic temperature patterns, seasonality and ocean circulation during this interval. We provide a chronologic framework for future studies analyzing ecological responses to profound climate change.

Virginia↗
USGS2021

Miocene neritic benthic foraminiferal community dynamics, Calvert Cliffs, Maryland, USA: Species pool, patterns and processes

The presence/absence and abundance of benthic foraminifera in successive discrete beds (Shattuck “zones”) of the Miocene Calvert and Choptank formations, exposed at the Calvert Cliffs, Maryland, USA, allows for investigation of community dynamics over space and time. The stratigraphic distribution of benthic foraminifera is documented and interpreted in the context of sea-level change, sequence stratigraphy, and the previously published distribution of mollusks. Neritic benthic foraminiferal communities of four sea-level cycles over ∼4 million years of the middle Miocene, encompassing the Miocene Climatic Optimum and the succeeding middle Miocene Climate Transition, are dominated by the same abundant species. They differ in the varying abundance of common species that occur throughout most of the studied section and in the different rare species that appear and disappear. Transgressive systems tracts (TSTs) have higher species diversity than highstand systems tracts (HSTs) but much lower density of specimens. In contrast to some previous research, all beds in the studied section are interpreted as being from the inner part of a broad, low gradient shelf and were deposited at water depths of less than ∼50 m. It is suggested that species are recruited from a regional species pool of propagules throughout the duration of TSTs. Recruitment is curtailed during highstands leading to lower diversity in the HSTs.

Maryland↗
USGS2021

Estimating Piacenzian sea surface temperature using an alkenone-calibrated transfer function

Stationarity of environmental preferences is a primary assumption required for any paleoenvironmental reconstruction using fossil materials based upon calibration to modern organisms. Confidence in this assumption decreases the further back in time one goes, and the validity of the assumption that species temperature tolerances have not changed over time has been challenged in Pliocene studies. We use paired U K′ 37 (unsaturated ketones with 37 carbon atoms) sea surface temperature (SST) and faunal assemblage data to directly calibrate North Atlantic Piacenzian planktonic foraminifer assemblages to Piacenzian alkenone paleotemperature estimates to provide an alternative paleoceanographic reconstruction approach that does not rely on stationarity. In doing so, we extend Pliocene SST estimates to sites where only quantitative faunal assemblage data were previously available and improve the spatial resolution of the North Atlantic SST reconstruction.

Scientific Investigations Report↗
USGS2021

Biogeography and ecology of Ostracoda in the U.S. northern Bering, Chukchi, and Beaufort Seas

Ostracoda (bivalved Crustacea) comprise a significant part of the benthic meiofauna in the Pacific-Arctic region, including more than 50 species, many with identifiable ecological tolerances. These species hold potential as useful indicators of past and future ecosystem changes. In this study, we examined benthic ostracodes from nearly 300 surface sediment samples, >34,000 specimens, from three regions—the northern Bering, Chukchi and Beaufort Seas—to establish species’ ecology and distribution. Samples were collected during various sampling programs from 1970 through 2018 on the continental shelves at 20 to ~100m water depth. Ordination analyses using species’ relative frequencies identified six species, Normanicythere leioderma , Sarsicytheridea bradii , Paracyprideis pseudopunctillata , Semicytherura complanata , Schizocythere ikeyai , and Munseyella mananensis , as having diagnostic habitat ranges in bottom water temperatures, salinities, sediment substrates and/or food sources. Species relative abundances and distributions can be used to infer past bottom environmental conditions in sediment archives for paleo-reconstructions and to characterize potential changes in Pacific-Arctic ecosystems in future sampling studies. Statistical analyses further showed ostracode assemblages grouped by the summer water masses influencing the area. Offshore-to-nearshore transects of samples across different water masses showed that complex water mass characteristics, such as bottom temperature, productivity, as well as sediment texture, influenced the relative frequencies of ostracode species over small spatial scales. On the larger biogeographic scale, synoptic ordination analyses showed dominant species— N . leioderma (Bering Sea), P . pseudopunctillata (offshore Chukchi and Beaufort Seas), and S . bradii (all regions)—remained fairly constant over recent decades. However, during 2013–2018, northern Pacific species M . mananensis and S . ikeyai increased in abundance by small but significant proportions in the Chukchi Sea region compared to earlier years. It is yet unclear if these assemblage changes signify a meiofaunal response to changing water mass properties and if this trend will continue in the future. Our new ecological data on ostracode species and biogeography suggest these hypotheses can be tested with future benthic monitoring efforts.

PLoS ONE↗
USGS2021

PlioMIP: The Pliocene Model Intercomparison Project

PlioMIP is a network of paleoclimate modelers and geoscientists who, through the study of the mid-Pliocene Warm Period (mPWP ~3.3–3.0 million years ago), seek to understand the sensitivity of the climate system to forcings and examine how well models reproduce past climate change.

Past Global Changes Magazine↗
USGS2020

Evaluation of Arctic warming in mid-Pliocene climate simulations

Palaeoclimate simulations improve our understanding of the climate, inform us about the performance of climate models in a different climate scenario, and help to identify robust features of the climate system. Here, we analyse Arctic warming in an ensemble of 16 simulations of the mid-Pliocene Warm Period (mPWP), derived from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). The PlioMIP2 ensemble simulates Arctic (60–90 ∘ N) annual mean surface air temperature (SAT) increases of 3.7 to 11.6 ∘ C compared to the pre-industrial period, with a multi-model mean (MMM) increase of 7.2 ∘ C. The Arctic warming amplification ratio relative to global SAT anomalies in the ensemble ranges from 1.8 to 3.1 (MMM is 2.3). Sea ice extent anomalies range from −3.0 to - 10.4 × 10 6 "> − 10.4 × 10 6 km 2 , with a MMM anomaly of - 5.6 × 10 6 "> − 5.6 × 10 6 km 2 , which constitutes a decrease of 53 % compared to the pre-industrial period. The majority (11 out of 16) of models simulate summer sea-ice-free conditions ( ≤ 1 × 10 6 "> ≤ 1 × 10 6 km 2 ) in their mPWP simulation. The ensemble tends to underestimate SAT in the Arctic when compared to available reconstructions, although the degree of underestimation varies strongly between the simulations. The simulations with the highest Arctic SAT anomalies tend to match the proxy dataset in its current form better. The ensemble shows some agreement with reconstructions of sea ice, particularly with regard to seasonal sea ice. Large uncertainties limit the confidence that can be placed in the findings and the compatibility of the different proxy datasets. We show that while reducing uncertainties in the reconstructions could decrease the SAT data–model discord substantially, further improvements are likely to be found in enhanced boundary conditions or model physics. Lastly, we compare the Arctic warming in the mPWP to projections of future Arctic warming and find that the PlioMIP2 ensemble simulates greater Arctic amplification than CMIP5 future climate simulations and an increase instead of a decrease in Atlantic Meridional Overturning Circulation (AMOC) strength compared to pre-industrial period. The results highlight the importance of slow feedbacks in equilibrium climate simulations, and that caution must be taken when using simulations of the mPWP as an analogue for future climate change.

Climate of the Past↗
USGS2019

Endless forams: >34,000 modern planktonic foraminiferal images for taxonomic training and automated species recognition using convolutional neural networks

Accurate planktonic foraminiferal species identification is central to many paleoceanographic studies, from selecting specific species for geochemical research to elucidating the biotic dynamics of microfossil communities relevant to physical oceanographic processes and interconnected phenomena such as climate change. However, species identification varies among taxonomic schools, few resources exist to train students in the difficult task of discerning amongst closely related species, and the number of taxonomic experts is limited. Here, we take the first steps towards removing these rate-limiting steps by generating the first extensive image library of modern planktonic foraminifera, providing taxonomic training tools and resources, and automating species-level taxonomic identification of planktonic foraminifera via machine learning using convolution neural networks. Taxonomic experts identified 34,640 images of modern planktonic foraminifera to the species level. These images are served as species exemplars through the online portal Endless Forams (endlessforams.org) and a taxonomic training portal hosted on the citizen science platform Zooniverse (zooniverse.org/projects/ahsiang/endless-forams/). A supervised machine learning classifier was then trained with more than 24,000 images of planktonic foraminifera and tested using the remaining ~10,000 images (i.e., the validation set). The best classifier provided the correct species name for an image in the validation set 87.4% of the time. Together, these resources provide a rigorous set of training tools in modern planktonic foraminiferal taxonomy and a means of rapidly generating assemblage data via machine learning in future studies.

Paleoceanography and Paleoclimatology↗
USGS2018

PRISM marine sites—The history of PRISM sea surface temperature estimation

For more than three decades, the U.S. Geological Survey (USGS) Pliocene Research, Interpretation and Synoptic Mapping (PRISM) Project has compiled paleoenvironmental data with the goal of reconstructing global conditions during the warm interval in the middle of the Piacenzian Age of the Pliocene Epoch (about 3.3 to 3.0 million years ago). Because this is the most recent interval of time in which climatic conditions were similar to those expected in the near future, a global reconstruction of conditions from this interval offers an imperfect yet useful representation of near future conditions. PRISM reconstructions have been used extensively as boundary conditions in general circulation model experiments aimed at better understanding Pliocene climate. They have also served as hindcasting targets when testing the ability of climate models to simulate real climates of the past, an exercise in estimating a model’s ability to accurately predict future climate. As data coverage has grown and model precision has improved, PRISM datasets have become important validation tools for pinpointing discrete areas of data-model disagreement and model-model disagreement. The Pliocene sea surface temperature (SST) dataset is the best developed component of the PRISM reconstructions and is the keystone of Pliocene paleoclimate research. For the first time, we compile all data related to PRISM SST estimation. This discussion chronicles the history of PRISM SST research as it evolved, responding to advances in paleochronology and paleotemperature estimation. Paleoclimatic considerations unique to each location are illustrated, as are any new developments since the initial publication of the data.

Open-File Report↗
USGS2018

Mid-piacenzian of the north Atlantic Ocean

The Piacenzian Age (Pliocene) represents a past climate interval within which frequency and magnitude of environmental changes during a period of past global warmth can be analyzed, climate models can be tested, and results can be placed in a context to better prepare for future change. Here we focus on the North Atlantic region, incorporating new and existing faunal assemblage and alkenone data from Ocean Drilling Program Sites 642, 662, 982, and 999, and International Ocean Discovery Program Sites 1308 and 1313 into our paleoenvironmental reconstruction. Cores and outcrop material containing Piacenzian sediments from the Atlantic Coastal Plain of Virginia, USA, are also included. These data allow us to characterize regional changes in temperature, salinity, upwelling, surface productivity, and diversity, associated with climate transitions, and make nuanced reconstructions of mid-Piacenzian conditions within a high-resolution temporal framework between ~3.40 and ~3.15 Ma, inclusive of Marine Isotope Stages M2 through KM5. We include an initial comparison of estimated sea-surface temperature to coupled climate model simulations, which shows improvement in model adherence to paleoclimate parameters over previous data-model comparisons for the Pliocene.

Stratigraphy↗
USGS2018

Icebergs in the Nordic Seas throughout the Late Pliocene

The Arctic cryosphere is changing and making a significant contribution to sea level rise. The Late Pliocene had similar CO2 levels to the present and a warming comparable to model predictions for the end of this century. However, the state of the Arctic cryosphere during the Pliocene remains poorly constrained. For the first time we combine outputs from a climate model with a thermodynamic iceberg model to simulate likely source regions for ice‐rafted debris (IRD) found in the Nordic Seas from Marine Isotope Stage M2 to the mid‐Piacenzian Warm Period and what this implies about the nature of the Arctic cryosphere at this time. We compare the fraction of melt given by the model scenarios with IRD data from four Ocean Drilling Program sites in the Nordic Seas. Sites 911A, 909C, and 907A show a persistent occurrence of IRD that model results suggest is consistent with permanent ice on Svalbard. Our results indicate that icebergs sourced from the east coast of Greenland do not reach the Nordic Seas sites during the warm Late Pliocene but instead travel south into the North Atlantic. In conclusion, we suggest a continuous occurrence of marine‐terminating glaciers on Svalbard and on East Greenland (due to the elevation of the East Greenland Mountains during the Late Pliocene). The study has highlighted the usefulness of coupled climate model‐iceberg trajectory modeling for understanding ice sheet behavior when proximal geological records for Pliocene ice presence or absence are absent or are inconclusive.

Paleoceanography↗
USGS2018

Sea surface temperature estimates for the mid-Piacenzian Indian Ocean—Ocean Drilling Program sites 709, 716, 722, 754, 757, 758, and 763

Despite the wealth of global paleoclimate data available for the warm period in the middle of the Piacenzian Stage of the Pliocene Epoch (about 3.3 to 3.0 million years ago [Ma]; Dowsett and others, 2013, and references therein), the Indian Ocean has remained a region of sparse geographic coverage in terms of microfossil analysis. In an effort to characterize the surface Indian Ocean during this interval, we examined the planktic foraminifera from Ocean Drilling Program (ODP) sites 709, 716, 722, 754, 757, 758, and 763, encompassing a wide range of oceanographic conditions. We quantitatively analyzed the data for sea surface temperature (SST) estimation using both the modern analog technique (MAT) and a factor analytic transfer function. The data will contribute to the U.S. Geological Survey (USGS) Pliocene Research, Interpretation and Synoptic Mapping (PRISM) Project’s global SST reconstruction and climate model SST boundary condition for the mid-Piacenzian and will become part of the PRISM verification dataset designed to ground-truth Pliocene climate model simulations (Dowsett and others, 2013).

Open-File Report↗
USGS2017

Emulation of long-term changes in global climate: application to the late Pliocene and future

Multi-millennial transient simulations of climate changes have a range of important applications, such as for investigating key geologic events and transitions for which high-resolution palaeoenvironmental proxy data are available, or for projecting the long-term impacts of future climate evolution on the performance of geological repositories for the disposal of radioactive wastes. However, due to the high computational requirements of current fully coupled general circulation models (GCMs), long-term simulations can generally only be performed with less complex models and/or at lower spatial resolution. In this study, we present novel longterm “continuous” projections of climate evolution based on the output from GCMs, via the use of a statistical emulator. The emulator is calibrated using ensembles of GCM simulations, which have varying orbital configurations and atmospheric CO2 concentrations and enables a variety of investigations of long-term climate change to be conducted, which would not be possible with other modelling techniques on the same temporal and spatial scales. To illustrate the potential applications, we apply the emulator to the late Pliocene (by modelling surface air temperature – SAT), comparing its results with palaeo-proxy data for a number of global sites, and to the next 200 kyr (thousand years) (by modelling SAT and precipitation). A range of CO2 scenarios are prescribed for each period. During the late Pliocene, we find that emulated SAT varies on an approximately precessional timescale, with evidence of increased obliquity response at times. A comparison of atmospheric CO2 concentration for this period, estimated using the proxy sea surface temperature (SST) data from different sites and emulator results, finds that relatively similar CO2 concentrations are estimated based on sites at lower latitudes, whereas higher-latitude sites show larger discrepancies. In our second illustrative application, spanning the next 200 kyr into the future, we find that SAT oscillations appear to be primarily influenced by obliquity for the first ∼ 120 kyr, whilst eccentricity is relatively low, after which precession plays a more dominant role. Conversely, variations in precipitation over the entire period demonstrate a strong precessional signal. Overall, we find that the emulator provides a useful and powerful tool for rapidly simulating the long-term evolution of climate, both past and future, due to its relatively high spatial resolution and relatively low computational cost. However, there are uncertainties associated with the approach used, including the inability of the emulator to capture deviations from a quasi-stationary response to the forcing, such as transient adjustments of the deep-ocean temperature and circulation, in addition to its limited range of fixed ice sheet configurations and its requirement for prescribed atmospheric CO2 concentrations.

Climate of the Past↗
USGS2016

The PRISM4 (mid-Piacenzian) paleoenvironmental reconstruction

The mid-Piacenzian is known as a period of relative warmth when compared to the present day. A comprehensive understanding of conditions during the Piacenzian serves as both a conceptual model and a source for boundary conditions as well as means of verification of global climate model experiments. In this paper we present the PRISM4 reconstruction, a paleoenvironmental reconstruction of the mid-Piacenzian ( ∼ 3 Ma) containing data for paleogeography, land and sea ice, sea-surface temperature, vegetation, soils, and lakes. Our retrodicted paleogeography takes into account glacial isostatic adjustments and changes in dynamic topography. Soils and lakes, both significant as land surface features, are introduced to the PRISM reconstruction for the first time. Sea-surface temperature and vegetation reconstructions are unchanged but now have confidence assessments. The PRISM4 reconstruction is being used as boundary condition data for the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) experiments.

Climate of the Past↗