Brady Z. Foreman

Autogenic geomorphic processes determine the resolution and fidelity of terrestrial paleoclimate records

Mesoscale geomorphic processes impose regularity in deposition, allowing quantitative resolution of proxy-based climate reconstructions. Terrestrial paleoclimate records rely on proxies hosted in alluvial strata whose beds are deposited by unsteady and nonlinear geomorphic processes. It is broadly assumed that this renders the resultant time series of terrestrial paleoclimatic variability noisy and incomplete. We evaluate this assumption using a model of oscillating climate and the precise topographic evolution of an experimental alluvial system. We find that geomorphic stochasticity can create aliasing in the time series and spurious climate signals, but these issues are eliminated when the period of climate oscillation is longer than a key time scale of internal dynamics in the geomorphic system. This emergent autogenic geomorphic behavior imparts regularity to deposition and represents a natural discretization interval of the continuous climate signal. We propose that this time scale in nature could be in excess of 104 years but would still allow assessments of the rates of climate change at resolutions finer than the existing age model techniques in isolation.

Anatomy, systematics, paleoenvironment, growth, and age of the sauropod dinosaur Sonorasaurus thompsoni from the Cretaceous of Arizona, USA

Abstract Sauropod dinosaurs are rare in the Cretaceous North American fossil record in general and are absent from that record for most of the Late Cretaceous. Sonorasaurus thompsoni from the Turney Ranch Formation of the Bisbee Group of Arizona, USA, potentially represents one of the youngest sauropods before their ca. 30-million-year-long hiatus from the record. The anatomy of Sonorasaurus has only been briefly described, its taxonomic validity has been questioned, several hypotheses have been proposed regarding its phylogenetic relationships, and its life history, geologic age, and reported paleoenvironment are ambiguous. Herein we assess the systematics, paleoenvironment, life history, and geologic age of Sonorasaurus based on firsthand observation, bone histology, and fieldwork in the holotypic quarry and environs. The validity of S. thompsoni is substantiated by autapomorphies. Cladistic analysis firmly places it within the Brachiosauridae, in contrast to results of some recent analyses. Bone histology suggests that the only known exemplar of Sonorasaurus grew slowly and sporadically compared to other sauropods and was approaching its adult size. In contrast with previous assessments of a coastal/estuarine paleoenvironment for the Turney Ranch Formation, our sedimentological and plant macrofossil data indicate that Sonorasaurus lived in a semiarid, low relief evergreen woodland that received highly variable (perhaps seasonal) precipitation. We obtained detrital zircons from the holotypic quarry for U-Pb dating, which only yielded Barremian-aged and older grains, whereas other radiometric and biostratigraphic data suggest that the sediments at the quarry were deposited near the Albian-Cenomanian boundary. Sonorasaurus is taxonomically valid, represents one of the geologically youngest brachiosaurid sauropods, and inhabited a harsh inland evergreen-dominated woodland environment that limited its growth. A review of other Bisbee Group dinosaurs suggests that its fauna, although poorly sampled, exhibits broad similarity to those from coeval North American horizons, reinforcing the apparent faunal homogeneity at the time.

Geomorphic stasis and spatiotemporal scales of stratigraphic completeness

Alluvial stratigraphic records are notoriously incomplete. All stratigraphic sections within sedimentary basins experience varying episodes of erosion and geomorphic stasis during their accumulation. This has detrimental effects on the completeness of paleoclimatic and paleobiologic records. Here we evaluate the resultant stratigraphic incompleteness using a physical experiment with self-organized vertical scales of topography and lateral scales of geomorphic stasis. First, we document how stratigraphic completeness improves as the temporal discretization interval coarsens, and show that the primary cause of the missing time shifts from stasis to erosion as the discretization is coarsened. Second, we demonstrate that the debilitating effect of finer temporal resolution can be predictably offset by compositing records across a wider area, and present a new two-dimensional formulation of stratigraphic completeness. These findings imply systematic shifts in taphonomic preservation, and by extension, the quality of paleobiologic and paleoclimatic proxy records.

Stream power controls the braiding intensity of submarine channels similarly to rivers

We use physical experiments to investigate the response of submarine braided channels driven by saline density currents to increasing inflow discharge and bed slope. We find that, similarly to braided rivers, only a fraction of submarine braided networks have active sediment transport. We then find similar response to imposed change between submarine and fluvial braided systems: (1) both the active and total braiding intensities increase with increasing discharge and slope; (2) the ratio of active braiding intensity to total braiding intensity is 0.5 in submarine braided systems regardless of discharge and slope; and (3) the active braiding intensity scales linearly with dimensionless stream power. Thus, braided submarine channels and braided rivers are similar in some important aspects of their behavior and responses to changes in stream power and bed slope. In light of the scale independence of braided channel planform organization, these results are likely to apply beyond experimental scales.

Estimating regional flood discharge during Palaeocene-Eocene global warming

Among the most urgent challenges in future climate change scenarios is accurately predicting the magnitude to which precipitation extremes will intensify. Analogous changes have been reported for an episode of millennial-scale 5 °C warming, termed the Palaeocene-Eocene Thermal Maximum (PETM; 56 Ma), providing independent constraints on hydrological response to global warming. However, quantifying hydrologic extremes during geologic global warming analogs has proven difficult. Here we show that water discharge increased by at least 1.35 and potentially up to 14 times during the early phase of the PETM in northern Spain. We base these estimates on analyses of channel dimensions, sediment grain size, and palaeochannel gradients across the early PETM, which is regionally marked by an abrupt transition from overbank palaeosol deposits to conglomeratic fluvial sequences. We infer that extreme floods and channel mobility quickly denuded surrounding soil-mantled landscapes, plausibly enhanced by regional vegetation decline, and exported enormous quantities of terrigenous material towards the ocean. These results support hypotheses that extreme rainfall events and associated risks of flooding increase with global warming at similar, but potentially at much higher, magnitudes than currently predicted.