Geology topics

Morgan Schaller

Publications and source records attributed to Morgan Schaller.

4 recordsLinked to original sources

USGS2026

Evaluating drivers of environmental change in a lake sediment core: Insights from spectroscopic metrics of water-extractable organic matter and stable carbon isotopes

Freshwater lakes play a critical role in the global carbon cycle by storing and transforming organic matter (OM) from both terrestrial and aquatic sources. Small lakes in northern temperate regions, despite their limited surface area, disproportionately influence regional carbon budgets. Buried sediments integrate OM inputs over time and archive ecosystem responses to natural and anthropogenic disturbances. However, the direction and magnitude of recent environmental changes on sediment carbon (C) dynamics remain poorly understood. A 23-cm core was collected from a small temperate lake in northeastern USA to evaluate sediment OM content and composition over timescales relevant to historical land-use change, damming, and recovery from acid deposition. Patterns in OM burial and source contributions were revealed via elemental and isotopic analyses of bulk OM and ultraviolet-visible spectrophotometry of water-extractable organic matter (WEOM). The optical metrics expanded observations of likely OM sources beyond the information gained from bulk carbon metrics (total carbon, δ 13 C). The aromaticity of WEOM increased downcore, which is consistent with a shift from increased terrestrial inputs during early logging and damming activity (pre ∼1920) to more microbial-derived OM in recent surficial sediments. Future applications of WEOM optical properties as complements to traditional geochemical metrics can enhance interpretations of lake ecosystem responses recorded in lake sediments to environmental perturbations in temperate lakes.

New York↗

USGS2024

Paleogene Earth perturbations in the US Atlantic Coastal Plain (PEP-US): Coring transects of hyperthermals to understand past carbon injections and ecosystem responses

The release of over 4500 Gt (gigatonnes) of carbon at the Paleocene–Eocene boundary provides the closest geological analog to modern anthropogenic CO 2 emissions. The cause(s) of and responses to the resulting Paleocene–Eocene Thermal Maximum (PETM) and attendant carbon isotopic excursion (CIE) remain enigmatic and intriguing despite over 30 years of intense study. CIE records from the deep sea are generally thin due to its short duration and slow sedimentation rates, and they are truncated due to corrosive bottom waters dissolving carbonate sediments. In contrast, PETM coastal plain sections along the US mid-Atlantic margin are thick, generally having an expanded record of the CIE. Drilling here presents an opportunity to study the PETM onset to a level of detail that could transform our understanding of this important event. Previous drilling in this region provided important insights, but existing cores are either depleted or contain stratigraphic gaps. New core material is needed for well-resolved marine climate records. To plan new drilling, members of the international scientific community attended a multi-staged, hybrid scientific drilling workshop in 2022 designed to maximize not only scientifically and demographically diverse participation but also to protect participants' health and safety during the global pandemic and to reduce our carbon footprint. The resulting plan identified 10 sites for drill holes that would penetrate the Cretaceous–Paleogene (K–Pg) boundary, targeting the pre-onset excursion (POE), the CIE onset, the rapidly deposited Marlboro Clay that records a very thick CIE body, and other Eocene hyperthermals. The workshop participants developed several primary scientific objectives related to investigating the nature and the cause(s) of the CIE onset as well as the biotic effects of the PETM on the paleoshelf. Additional objectives focus on the evidence for widespread wildfires and changes in the hydrological cycle, shelf morphology, and sea level during the PETM as well as the desire to study both underlying K–Pg sediments and overlying post-Eocene records of extreme hyperthermal climate events. All objectives address our overarching research question: what was the Earth system response to a rapid carbon cycle perturbation?

Scientific Drilling↗

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↗

USGS2019

Interglacial paleoclimate in the Arctic

Marine Isotope Stage 11 from ~424 to 374 ka experienced peak interglacial warmth and highest global sea level ~410–400 ka. MIS 11 has received extensive study on the causes of its long duration and warmer than Holocene climate, which is anomalous in the last half million years. However, a major geographic gap in MIS 11 proxy records exists in the Arctic Ocean where fragmentary evidence exists for a seasonally sea ice-free summers and high sea-surface temperatures (SST; ~8–10 °C near the Mendeleev Ridge). We investigated MIS 11 in the western and central Arctic Ocean using 12 piston cores and several shorter cores using proxies for surface productivity (microfossil density), bottom water temperature (magnesium/calcium ratios), the proportion of Arctic Ocean Deep Water versus Arctic Intermediate Water (key ostracode species), sea ice (epipelagic sea ice dwelling ostracode abundance), and SST (planktic foraminifers). We produced a new benthic foraminiferal δ 18 O curve, which signifies changes in global ice volume, Arctic Ocean bottom temperature, and perhaps local oceanographic changes. Results indicate that peak warmth occurred in the Amerasian Basin during the middle of MIS 11 roughly from 410 to 400 ka. SST were as high as 8–10 °C for peak interglacial warmth, and sea ice was absent in summers. Evidence also exists for abrupt suborbital events punctuating the MIS 12-MIS 11-MIS 10 interval. These fluctuations in productivity, bottom water temperature, and deep and intermediate water masses (Arctic Ocean Deep Water and Arctic Intermediate Water) may represent Heinrich-like events possibly involving extensive ice shelves extending off Laurentide and Fennoscandian Ice Sheets bordering the Arctic.

Paleoceanography and Paleoclimatology↗