Alexander R. Simms

Progressive Cenozoic cooling and the demise of Antarctica’s last refugium

The Antarctic Peninsula is considered to be the last region of Antarctica to have been fully glaciated as a result of Cenozoic climatic cooling. As such, it was likely the last refugium for plants and animals that had inhabited the continent since it separated from the Gondwana supercontinent. Drill cores and seismic data acquired during two cruises (SHALDRIL I and II) in the northernmost Peninsula region yield a record that, when combined with existing data, indicates progressive cooling and associated changes in terrestrial vegetation over the course of the past 37 million years. Mountain glaciation began in the latest Eocene (approximately 37–34 Ma), contemporaneous with glaciation elsewhere on the continent and a reduction in atmospheric CO2 concentrations. This climate cooling was accompanied by a decrease in diversity of the angiosperm-dominated vegetation that inhabited the northern peninsula during the Eocene. A mosaic of southern beech and conifer-dominated woodlands and tundra continued to occupy the region during the Oligocene (approximately 34–23 Ma). By the middle Miocene (approximately 16–11.6 Ma), localized pockets of limited tundra still existed at least until 12.8 Ma. The transition from temperate, alpine glaciation to a dynamic, polythermal ice sheet took place during the middle Miocene. The northernmost Peninsula was overridden by an ice sheet in the early Pliocene (approximately 5.3–3.6 Ma). The long cooling history of the peninsula is consistent with the extended timescales of tectonic evolution of the Antarctic margin, involving the opening of ocean passageways and associated establishment of circumpolar circulation.

Coastal Impact Underestimated From Rapid Sea Level Rise

A primary effect of global warming is accelerated sea level rise, which will eventually drown low-lying coastal areas, including some of the worlds most populated cities. Predictions from the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) suggest that sea level may rise by as much as 0.6 meter by 2100 [Solomon et al., 2007]. However, uncertainty remains about how projected melting of the Greenland and Antarctic ice sheets will contribute to sea level rise. Further, considerable variability is introduced to these calculations due to coastal subsidence, especially along the northern Gulf of Mexico (see http://tidesandcurrents.noaa.gov/sltrends/sltrends.shtml).

Holocene sea-level change derived from microbial mats

Few submeter-resolution relative sea-level (RSL) proxies exist for semiarid and arid coastlines that lack well-developed marshes or coral reefs. Because 15% of the world’s nonpolar coastline is in desert and steppe precipitation regimes, a new RSL proxy is needed for these regions. In this study we show the utility of microbial mats, a common biota found along semiarid and arid coastlines, as an RSL proxy. The indicative range of microbial mats in Baffin Bay (Texas, United States) is ±0.29 m, much less than the ±2 m indicative range of typical sea-level indicators currently used along the semiarid Texas coast. The elevations of 22 buried radiocarbon-dated microbial mats plot within error of RSL data derived from the central Texas coast for the past 5.0 k.y., suggesting that microbial mats provide a robust proxy for paleo–sea levels along semiarid and arid coastlines.

Geomorphology and age of the Oxygen isotope stage 2 (last lowstand) sequence boundary on the northwestern Gulf of Mexico continental shelf

Abstract The sequence boundary associated with the last glacial-eustatic lowstand was mapped across the northwestern Gulf of Mexico continental shelf. The geomorphology of incised fluvial valleys varies widely across the shelf. These differences are due to differences in shelf physiography and the interval of the eustatic cycle the valleys were occupied. Incision begins during the falling limb of sea level and results in terraced valleys. Rivers that abandoned their valleys during the fall in sea level to cut new valleys during the lowstand generally have u-shaped profiles. Incised valleys connected to turbidite systems only occurred in two valleys (the Colorado and Rio Grande), but this may be because sea level did not fall below the shelf break during the last eustatic cycle. Some valleys deepen in an offshore direction, others become shallower. The timing of fluvial incision was constrained using radiocarbon dates so that incision can be tied directly to the sea-level curve for the last glacial-eustatic cycle. The results show that the fluvial incision occurred throughout the falling limb of sea level and lowstand; however, maximum incision occurred during the lowest position of sea level. The resulting surface has significant relief, extends across the shelf, and has time significance. The associated conformable surface, on the other hand, is much harder to recognize and occurs at different stratigraphic levels relative to different shelf-margin deltas.

Quantifying the distribution of nanodiamonds in pre-Younger Dryas to recent age deposits along Bull Creek, Oklahoma panhandle, USA.

Significance In 2007, scientists proposed that the start of the Younger Dryas (YD) chronozone (10,900 radiocarbon years ago) and late Pleistocene extinctions resulted from the explosion of a comet in the earth’s atmosphere. The ET event, as it is known, is purportedly marked by high levels of various materials, including nanodiamonds. Nanodiamonds had previously been reported from the Bull Creek, Oklahoma, area. We investigate this claim here by quantifying the distribution of nanodiamonds in sediments of different periods within the Bull Creek valley. We found high levels of nanodiamonds in YD boundary deposits, supporting the previous claim. A second spike in nanodiamonds during the late Holocene suggests that the distribution of nanodiamonds is not unique to the YD. High levels of nanodiamonds (nds) have been used to support the transformative hypothesis that an extraterrestrial (ET) event (comet explosion) triggered Younger Dryas changes in temperature, flora and fauna assemblages, and human adaptations [Firestone RB, et al. (2007) Proc Natl Acad Sci USA 104(41):16016–16021]. We evaluate this hypothesis by establishing the distribution of nds within the Bull Creek drainage of the Beaver River basin in the Oklahoma panhandle. The earlier report of an abundance spike of nds in the Bull Creek I Younger Dryas boundary soil is confirmed, although no pure cubic diamonds were identified. The lack of hexagonal nds suggests Bull Creek I is not near any impact site. Potential hexagonal nds at Bull Creek were found to be more consistent with graphene/graphane. An additional nd spike is found in deposits of late Holocene through the modern age, indicating nds are not unique to the Younger Dryas boundary. Nd distributions do not correlate with depositional environment, pedogenesis, climate perturbations, periods of surface stability, or cultural activity.

Where do coastlines stabilize following rapid retreat

We present a numerical model that shows that the transgressing upper shoreline of wave-dominated estuaries (bayhead deltas), which commonly contain populous urban and industrial centers, stabilizes, and their rate of retreat decreases at tributary junctions. The decreased rate of retreat across a tributary junction is caused by a decrease in the total accommodation, while sediment supply remains conserved. Our model predicts that bayhead deltas from smaller systems will be located closer to tributary confluences than their larger counterparts. An examination of the modern bayhead deltas in Albemarle Sound, U.S. Atlantic Coast, reveals that bayhead deltas from smaller tributaries are located closer to tributary confluences than bayhead deltas associated with larger tributaries, supporting our model prediction. Our results highlight the importance of antecedent topography created during falling sea-levels on shaping the nature of transgression during the ensuing sea-level rise. In particular, tributary junctions act as pinning points during transgression.

Marine Radiocarbon Reservoir Values in Southern California Estuaries: Interspecies, Latitudinal, and Interannual Variability

Many studies use radiocarbon dates on estuarine shell material to build age-depth models of sediment accumulation in estuaries in California, USA. Marine 14 C ages are typically older than dates from contemporaneous terrestrial carbon and local offsets (∆R) from the global average marine offset need to be calculated to ensure the accuracy of calibrated dates. We used accelerator mass spectrometry (AMS) 14 C dating on 40 pre-1950 salt marsh snail and clam shells previously collected from four California estuaries. The average ∆R and standard deviation of 217 ± 129 14 C yr is consistent with previous calculations using mixed estuarine and marine samples, although the standard deviation and resulting age uncertainty was higher for our estuarine calculations than previous studies. There was a slight but significant difference ( p = 0.024) in ∆R between epifaunal snails (∆R = 171 ± 154 14 C yr) and infaunal clams (∆R = 263 ± 77 14 C yr), as well as between samples from individual estuaries. However, a closer examination of the data shows that even for the same species, at the same estuary, ∆R can vary as much as ~500 14 C yr. In some cases, the bulk of this variation occurs between samples collected by different collectors at different times, potentially indicating time dependence in carbon sources and ∆R variation. These variations could also be attributed to differences in collection location within a single estuary and resulting spatial differences in carbon sources. Intertidal specimens located in the high marsh may have lower ∆R than fully marine counterparts because of increased terrestrial 14 C input. The large variations in ∆R here highlight the need for conservative chronological interpretations, as well as the assumption of wide uncertainties, when dating samples from estuarine sources. DOI: 10.2458/azu_rc.57.18389

Marine terraces and rates of vertical tectonic motion: The importance of glacio-isostatic adjustment along the Pacific coast of central North America

Differences in marine terrace elevations across the Pacific coast of North America have long been assumed to be a result of differences in the rates of tectonic motion. However, other processes, particularly glacio-isostatic adjustment, lead to regional variations in sea levels. In this study, we compiled the elevations of marine isotope stage (MIS) 5e (ca. 119–129 ka), 5c (ca. 106 ka), and 5a (ca. 84 ka) terraces across the Pacific coast of central North America and compared these regional variations in elevation with model predictions of glacio-isostatic adjustment after correcting for tectonics. These predictions are generally consistent with the observed trends in the elevations of the terraces and show that this process created up to 20 m of coeval variation in local sea levels along the Pacific coast of central North America (between 20°N and 45°N) during MIS 5c and MIS 5a, but less, ~4 m, during MIS 5e. Accounting for glacio-isostatic adjustment reduces the variability in uplift rates calculated at individual locations using different-aged terraces as datums. Ignoring glacio-isostatic adjustment leads to overestimated uplift rates by an average of 40%, but up to 72%, across the Pacific coast of central North America. An understanding of regional variations in glacio-isostatic adjustment–corrected sea levels also contributes to the correct identification of marine terraces with mistaken ages.

Constraints on Antarctic Ice Sheet configuration during and following the Last Glacial Maximum and its episodic contribution to sea-level rise

Abstract Marine geological studies provide a record of diachronous expansion and retreat of the Antarctic Peninsula Ice Sheet, West Antarctic Ice Sheet and East Antarctic Ice Sheet during the past c. 30 000 cal yr BP. Retreat of these ice sheets and Antarctica’s contribution to sea-level rise was largely complete by the early Holocene. Estimates of ice sheet thickness, based on maximum grounding depths, range from 640 to 1640 m on the inner continental shelf. Grounding depths on the outer continental shelf equate to minimum thicknesses of 410–950 m. Geomorphic features indicate that retreat from the continental shelf was mostly step-wise around the continent, a result of the different factors that control ice sheet behaviour and the degree to which these factors vary regionally. Thus, the nature of post-LGM (Last Glacial Maximum) sea-level rise was episodic and believed to have been punctuated by rapid pulses triggered by individual ice stream collapse. Most of these pulses would have been of sub-metre magnitudes and below the resolution of existing sea-level curves, but they would have had significant impact on coastal evolution, especially along low-gradient coasts.

The role of buoyancy reversal in turbidite deposition and submarine fan geometry

Although recent work has shown that changing interstitial fluid density within turbidity currents is a frequently overlooked factor affecting the texture and internal architecture of turbidites, little is known about its influence on submarine fan morphology. Here we present the results of three-dimensional flume experiments of turbidity currents that clearly demonstrate the role of low-density interstitial fluid, in combination with sediment concentration and basin gradient, on submarine fan geometry. The experiments show that turbidity currents with reversing buoyancy, and their resulting deposits, are narrower than those that remain ground hugging. Furthermore, wider deposits result from increases in sediment concentration and/or basin-floor gradient. We also propose that Taylor-Gortler vortices associated with currents traveling over a break in slope may lead to the deposition of wider lobes compared with those traveling over a constant gradient.