Roland Mundil’s research while affiliated with Berkeley Geochronology Center and other places

The middle Cretaceous greenhouse period experienced profound environmental change including episodes of enhanced global burial of organic carbon marked by carbon isotopic excursions (CIEs). However, the role and response of polar regions like the newly formed, partially enclosed Arctic Ocean Basin during middle Cretaceous carbon burial remains enigmatic. We present the first Arctic deepwater CIE record that characterizes conditions offshore of the Alaska margin north of 75 • N paleolatitude. Organic carbon isotopes (δ 13 C org) and 103-82 Ma ash zircon U-Pb dates from the distal Hue Shale record multiple Albian-Campanian CIEs during slow ~3-15 m/Myr sediment accumulation rates. Average total organic carbon (TOC) increased substantially during large 2-3 ‰ CIEs of the ~101 Ma Albian-Cenomanian boundary event (from 7 to 18 % TOC) and ~94 Ma Cenomanian-Turonian boundary event (5 to 10 % TOC). Turonian TOC remained elevated (8-13 %) during high global sea levels and temperatures of the Cretaceous Thermal Maximum, followed by an increase from 7 to 11 % TOC during the ~90 Ma late Turonian event 1.5 ‰ CIE. Average TOC subsequently decreased in the Con-iacian-Campanian, but relative maxima occurred during subtle 0.5-1 ‰ CIEs interpreted as the ~87 Ma late Coniacian event (increase from 4 to 7 % TOC), ~85 Ma Horseshoe Bay event (3.5 to 4.5 % TOC), and ~84 Ma Santonian-Campanian boundary event (3.5 to 5 % TOC). Increases in hydrogen index and productivity proxies (P, Ba, Nd) that accompanied each CIE episode with enhanced TOC suggest a strong link between marine productivity and organic carbon burial at short-term CIE timescales. However, long-term (>5-8 Myr) changes in trace metal redox (Mo, Fe, V) and salinity (B/Ga) proxies suggest shifts in prevailing environmental conditions at timescales longer than the CIEs. Late Albian-middle Turonian marine salinity occurred during euxinic (103-98 Ma) and suboxic (98-90 Ma) conditions with deposition interpreted to have occurred within and beneath an oxygen minimum zone, respectively. In contrast, late Turonian-early Campanian (90-82 Ma) freshening and restricted euxinic basin conditions may signal the start of widespread restriction known to characterize the Paleogene Arctic. Overall, these results highlight that middle Cretaceous Arctic deepwater remained a productive marine carbon sink coupled to the global carbon cycle despite evolving Arctic greenhouse conditions.

Triassic non-marine strata from Gondwana are dated almost exclusively by biostratigraphic means, given the rarity of precise radioisotopic and magnetostratigraphic datasets. A primary method for constraining the age of these sedimentary sequences is palynomorph biostratigraphy, which implicitly assumes that differences in the presence/absence of taxa between two or more assemblages reflect a difference in geological ages. But without biostratigraphically-independent age data, this assumption often remains untested. The Triassic El Peñasco Group of the Santa Clara sub-basin in the Cuyana Basin of central-west Argentina is an excellent study system to help test these assumptions, because the upper part of the sequence comprises a fluvio-deltaic-lacustrine succession that preserves volcaniclastic horizons and extensive palynomorph assemblages. Previous palynostratigraphic analyses had inferred a Late Triassic (Carnian-Norian) age for the Santa Clara Abajo and overlying Santa Clara Arriba formations, but we present new precise U-Pb CA-TIMS zircon ages that suggest an upper Anisian (Middle Triassic) age for both formations. This makes it the only non-marine Anisian Gondwanan sequence, and one of only a handful globally, to be precisely dated geochronologically. These new ages also constrain the depo-sition of the >700 m-thick succession of the Santa Clara Abajo and Santa Clara Arriba formations to no more than 2.1 my of time, suggesting it represents a synrift phase of the sub-basin. Combining these and existing geochronologic data with a newly assembled comprehensive presence/absence dataset of palynomorphs from the Anisian-Norian of Gondwana, we demonstrate that paleogeography (paleolati-tude) has a significantly stronger correlation with taxonomic composition of assemblages than does geo-logic time. These results imply that geography is an important null hypothesis in explaining differences in early Mesozoic Gondwanan palynomorph assemblages, and that precise geochronologic age constraints are important for refining the accuracy of Triassic palynomorph biochronology.

Icehouse climate systems occur across an abbreviated portion of Earth history, comprising ∼25% of the Phanerozoic record. The Late Paleozoic Ice Age (LPIA) was the most extreme and longest lasting glaciation of the Phanerozoic and is characterized by periods of acute continental scale glaciation, separated by periods of ice minima or ice-free conditions on the order of <10 ⁶ years. The late Paleozoic glaciogenic record of the Paraná and Kalahari basins of southern Gondwana form one of the largest, best preserved, and well calibrated records of this glaciation. In the Carboniferous, the eastern and southern margins of the Paraná Basin and the Kalahari Basin were characterized by subglacial conditions, with evidence for continental and upland glaciers. In the latest Carboniferous, these basins transitioned from subglacial reservoirs to ice-free conditions evidenced by the widespread deposition of marine deposits juxtaposed on subglacial bedforms. High-precision U-Pb zircon CA-TIMS geochronologic constraints from volcanic ash deposits in the deglacial marine black shales of the Kalahari Basin and from fluvial and coal successions, which overlie marine deposits in the Paraná, indicate subglacial evidence in these regions is constrained to the Carboniferous. The loss of ice in these regions is congruent with a late Carboniferous peak in p CO 2 and widespread marine anoxia in the late Carboniferous. The permeant retreat of glaciers in basinal settings, despite an early Permian p CO 2 nadir, highlight the influence of short-term perturbations on the longer-term CO 2 record and suggests an ice-threshold had been crossed in the latest Carboniferous. A definitive driver for greenhouse gases in the LPIA, such as abundant and sustained volcanic activity or an increased biologic pump driven by ocean fertilization, is unresolved for this period. Lastly, the proposed Carboniferous apex for the high-latitude LPIA record is incongruent with observations from the low-latitude tropics where an early Permian peak is proposed.