Geology topics

Robert Poirier

Publications and source records attributed to Robert Poirier.

5 recordsLinked to original sources

USGS2023

A 600-kyr reconstruction of deep Arctic seawater δ18O from benthic foraminiferal oxygen isotopes and ostracode Mg/Ca paleothermometry

The oxygen isotopic composition of benthic foraminiferal tests ( δ 18 O b ) is one of the pre-eminent tools for correlating marine sediments and interpreting past terrestrial ice volume and deep-ocean temperatures. Despite the prevalence of δ 18 O b applications to marine sediment cores over the Quaternary, its use is limited in the Arctic Ocean because of low benthic foraminiferal abundances, challenges with constructing independent sediment core age models, and an apparent muted amplitude of Arctic δ 18 O b variability compared to open-ocean records. Here we evaluate the controls on Arctic δ 18 O b by using ostracode Mg / Ca "> paleothermometry to generate a composite record of the δ 18 O of seawater ( δ 18 O sw ) from 12 sediment cores in the intermediate to deep Arctic Ocean (700–2700 m) that covers the last 600 kyr based on biostratigraphy and orbitally tuned age models. Results show that Arctic δ 18 O b was generally higher than open-ocean δ 18 O b during interglacials but was generally equivalent to global reference records during glacial periods. The reduced glacial–interglacial Arctic δ 18 O b range resulted in part from the opposing effect of temperature, with intermediate to deep Arctic warming during glacials counteracting the whole-ocean δ 18 O sw increase from expanded terrestrial ice sheets. After removing the temperature effect from δ 18 O b , we find that the intermediate to deep Arctic experienced large ( ≥1 ‰) variations in local δ 18 O sw , with generally higher local δ 18 O sw during interglacials and lower δ 18 O sw during glacials. Both the magnitude and timing of low local δ 18 O sw intervals are inconsistent with the recent proposal of freshwater intervals in the Arctic Ocean during past glaciations. Instead, we suggest that lower local δ 18 O sw in the intermediate to deep Arctic Ocean during glaciations reflected weaker upper-ocean stratification and more efficient transport of low- δ 18 O sw Arctic surface waters to depth by mixing and/or brine rejection.

Climate of the Past↗

USGS2023

Roles of climatic and anthropogenic factors in shaping Holocene vegetation and fire regimes in Great Dismal Swamp, eastern USA

The Great Dismal Swamp wetland, spanning >400 km 2 along the Virginia and North Carolina border, was shaped by a complex combination of geomorphic, climatic, and anthropogenic forcings during the last 14,000 years. Pollen, macrofossils, charcoal, and physical properties from sediment cores at seven sites provide a detailed record of the spatial heterogeneity of the wetland and the roles played by natural hydrologic variability, wildfire, and human modification of drainage in shaping vegetation and habitats. Cold-temperate forests occupied regional uplands from at least 13.5–10.3 cal ka BP. Marshes dominated by grasses and other herbaceous taxa began developing along low-elevation streams as early as 10.3 cal ka BP, resulting in accumulation of organic silts. Long-hydroperiod, peat accumulating marshes, with abundant floating aquatic plants, developed as early as 9.6 cal ka BP, as rapid rates of sea-level rise elevated the water table and facilitated wetland development and peat accumulation along stream courses. By the mid-Holocene (c. 7–6.5 cal ka BP), when local sea-level rise began slowing and reached about 12–15 m below present, shorter hydroperiod, peat-accumulating marshes dominated the landscape, with increased wildfire activity. Great Dismal Swamp vegetation shifted from marshes to peat-accumulating forested wetlands by c. 3.7 cal ka BP; these were dominated by varying combinations of Nyssa (tupelo), Taxodium (cypress), and Chamaecyparis thyoides (Atlantic white cedar). Wildfires were infrequent during this time, and the forested wetlands persisted, with minor compositional changes related to climate-driven fluctuations in stream flow, until colonial ditching and logging began in the swamp during the late 18th century. These activities decreased cypress and cedar populations, and, by the mid-20th century, expanded ditching resulted in even drier conditions and expansion of maple-gum (dominated by Acer and Liquidambar ) , and pine-pocosin (dominated by Pinus ) forests. The distribution of these forests differs from that of the late Holocene and represents a fundamental shift in hydrology, peat structure, vegetation, and fire regime due to landscape alterations of the last few centuries.

North Carolina, Virginia↗

USGS2023

A new deglacial climate and sea-level record from 20 to 8 ka from IODP381 site M0080, Alkyonides Gulf, eastern Mediterranean

Records of relative sea-level rise for the last deglaciation are mostly limited to coral reef records and geophysical model estimates, but observational data from regions with temperate climates is sparse. We present a new relative climatic and regional sea-level rise record for glacial Termination 1 (Marine Isotope Stages [MIS] 2–1) based on ostracode paleoecology from the upper 8 m of the International Ocean Discovery Program (IODP) Site M0080 collected on Expedition 381, in the Gulf of Alkyonides, eastern Corinth basin of the Mediterranean Sea. Results show a series of major faunal transitions from lacustrine (Ponto-Caspian, Lake Corinth) glacial-age assemblages to fully marine (Mediterranean) interglacial assemblages between 20 and 8 ka. During glacial and early deglacial intervals, the Gulf of Alkyonides was characterized by non-marine lacustrine conditions with episodic sediment input from coastal, saline lake environments. Relatively stable lake shoreline conditions marked by the distinctive Tuberoloxoconcha sp. Existed from ∼17.5 to 15 ka. During the peak deglacial interval, the BØlling-AllerØd (B-A, ∼15–13.5 ka), rapid sea-level rise is indicated by a fully marine ostracode fauna colonization, which persisted from 13.5 to 7.5 ka (Late Pleistocene-Early to Middle Holocene).

Quaternary Science Reviews↗

USGS2022

Abrupt quaternary ocean-ice events in the Arctic: Evidence from the ostracode rabilimis

The Arctic Ocean has experienced orbital and millennial-scale climate oscillations over the last 500 kilo-annum (ka) involving massive changes in global sea level and components of the Arctic cryosphere, including sea-ice cover, land-based ice sheets and ice shelves. Although these climate events are only partially understood, micropaleontological studies utilizing ostracodes and benthic foraminifera have demonstrated that major changes in faunas have occurred at different timescales that signify ecosystem regime changes linked to sea-ice cover, surface productivity, bottom temperature and other factors. In addition to faunal changes characterizing glacial-interglacial cycles, Arctic sediments contain several unusual faunal events that cannot be explained by orbital-scale sea level and cryospheric changes. One indicator of such events involves the ostracode Rabilimis mirabilis (Brady 1868), a shallow-water species that inhabits continental shelves in the modern Arctic. We conducted studies of the stratigraphic distribution of R. mirabilis in cores from the Northwind, Mendeleev, Lomonosov, and Alpha Ridges; the Siberian and North American (Beaufort Sea) continental margins; and the Lincoln Sea off North Greenland and in the northern Greenland Sherard Osborn Fjord. Evidence from these records suggests that this species occurs as a fossil in deeper water sediment cores on the upper parts of submarine ridges (mainly 700-900 meters water depth, mwd), in significant numbers (from 1%to 50% of total ostracodes) during Marine Isotope Stages (MIS) 5a (125-109 ka), MIS 4 (71-57 ka), and MIS 3 (57-29 ka). Furthermore, it occurs in cores from various depths on the Siberian margin, the Beaufort and Lincoln Seas during MIS 1 (the Holocene, approx. 11-0 ka). These occurrences involve well-preserved, stratigraphically consistent adult and juvenile populations, which are autochthonous in nature and not caused by downslope transport or ice rafting. Based on their age and associated paleoceanographic conditions in the Arctic, we interpret these R. mirabilis events as signifying basin-ward migration during abrupt changes in growth and decay of massive ice shelves and may be useful as biostratigraphic markers.

Micropaleontology↗

USGS2021

Quantifying diagenesis, contributing factors, and resulting isotopic bias in benthic foraminifera using the Foraminiferal Preservation Index: Implications for geochemical proxy records

Geochemical records generated from the calcite tests of benthic foraminifera, especially those of the genera Cibicidoides and Uvigerina , provide the basis for proxy reconstructions of past climate. However, the extent to which benthic foraminifera are affected by postdepositional alteration is poorly constrained. Furthermore, how diagenesis may alter the geochemical composition of benthic foraminiferal tests, and thereby biasing a variety of proxy-based climate records, is also poorly constrained. We present the Foraminiferal Preservation Index (FPI) as a new metric to quantify preservation quality based on objective, well-defined criteria. The FPI is used to identify and quantify trends in diagenesis temporally, from late Pliocene to modern coretop samples (3.3–0 Ma), as well as spatially in the deep ocean. The FPI identifies the chemical composition of deep-ocean water masses to be the primary driver of diagenesis through time, while also serving as a supplementary method of identifying periods of changing water mass influence at a given site. Additionally, we present stable isotope data (δ 18 O, δ 13 C) generated from individual Cibicidoides specimens of various preservation quality that demonstrate the likelihood of significant biasing in a variety of geochemical proxy records, especially those used to reconstruct past changes in ice volume and sea level. These single-test data further demonstrate that when incorporating carefully selected tests of only the highest preservation quality, robust paleorecords can be generated.

Paleoceanography and Paleoclimatology↗