G. Burch Fisher

Fallout plume of submerged oil from Deepwater Horizon

Significance Following the sinking of the Deepwater Horizon in the Gulf of Mexico an unprecedented quantity of oil irrupted into the ocean at a depth of 1.5 km. The novelty of this event makes the oil’s subsequent fate in the deep ocean difficult to predict. This work identifies a fallout plume of hydrocarbons from the Macondo Well contaminating the ocean floor over an area of 3,200 km2. Our analysis suggests the oil initially was suspended in deep waters and then settled to the underlying sea floor. The spatial distribution of contamination implicates accelerated settling as an important fate for suspended oil, supports a patchwork mosaic model of oil deposition, and frames ongoing attempts to determine the event’s impact on deep-ocean ecology. The sinking of the Deepwater Horizon in the Gulf of Mexico led to uncontrolled emission of oil to the ocean, with an official government estimate of ∼5.0 million barrels released. Among the pressing uncertainties surrounding this event is the fate of ∼2 million barrels of submerged oil thought to have been trapped in deep-ocean intrusion layers at depths of ∼1,000–1,300 m. Here we use chemical distributions of hydrocarbons in >3,000 sediment samples from 534 locations to describe a footprint of oil deposited on the deep-ocean floor. Using a recalcitrant biomarker of crude oil, 17α(H),21β(H)-hopane (hopane), we have identified a 3,200-km2 region around the Macondo Well contaminated by ∼1.8 ± 1.0 × 106 g of excess hopane. Based on spatial, chemical, oceanographic, and mass balance considerations, we calculate that this contamination represents 4–31% of the oil sequestered in the deep ocean. The pattern of contamination points to deep-ocean intrusion layers as the source and is most consistent with dual modes of deposition: a “bathtub ring” formed from an oil-rich layer of water impinging laterally upon the continental slope (at a depth of ∼900–1,300 m) and a higher-flux “fallout plume” where suspended oil particles sank to underlying sediment (at a depth of ∼1,300–1,700 m). We also suggest that a significant quantity of oil was deposited on the ocean floor outside this area but so far has evaded detection because of its heterogeneous spatial distribution.

Dominance of tectonics over climate in himalayan denudation

Landscape denudation in actively deforming mountain ranges is controlled by a combination of rock uplift and surface runoff induced by precipitation. Whereas the relative contribution of these factors is important to our understanding of the evolution of orogenic topography, no consensus currently exists concerning their respective infl uences. To address this question, denudation rates at centennial to millennial time scales were deduced from 10Be concentrations in detrital sediments derived from 30 small basins (10-600 km2) in an ~200-km-wide region in central Nepal. Along a northward, strike-perpendicular transect, average denudation rates sharply increase from <0.5 mm/yr in the Lesser Himalayas to ~1 mm/yr when crossing the Physiographic Transition, and then accelerate to 2-3 mm/yr on the southern flank of the high peaks in the Greater Himalayas. Despite a more than five-fold increase in denudation rate between the southern and northern parts of this transect, the corresponding areas display similar precipitation rates. The primary parameter that presents a signifi cant co-variation with denudation is the long-term rockuplift rate that is interpreted to result from the ramp-fl at transition along the Main Himalayan Thrust. We propose that, in this rapidly uplifting mountain range, landscapes adjust quickly to changing climatic conditions, such that denudation is mainly limited by the rate at which material is pushed upward by tectonic processes and made available for removal by surface processes. In this particular context, variations in precipitation appear to have only a second-order role in modulating the denudation signal that is primarily set by the background rock-uplift rate.

Persistence and biodegradation of oil at the ocean floor following Deepwater Horizon

Significance The Deepwater Horizon event led to an unprecedented discharge of ∼4.1 million barrels of oil to the Gulf of Mexico. The deposition of ∼4–31% of this oil to the seafloor has been quantified previously on a bulk basis. In this work, we assess the extent of degradation over 4 y postspill for each of 125 petroleum hydrocarbons that contaminated the seafloor. As expected, chemically simpler compounds broke down more quickly than complex compounds, but degradation rates also depended on environmental context: Breakdown often was faster before seafloor deposition than after and for oil trapped in small droplets than for oil in large particles. These results provide a basis to predict the long-term fate of seafloor oil. The 2010 Deepwater Horizon disaster introduced an unprecedented discharge of oil into the deep Gulf of Mexico. Considerable uncertainty has persisted regarding the oil’s fate and effects in the deep ocean. In this work we assess the compound-specific rates of biodegradation for 125 aliphatic, aromatic, and biomarker petroleum hydrocarbons that settled to the deep ocean floor following release from the damaged Macondo Well. Based on a dataset comprising measurements of up to 168 distinct hydrocarbon analytes in 2,980 sediment samples collected within 4 y of the spill, we develop a Macondo oil “fingerprint” and conservatively identify a subset of 312 surficial samples consistent with contamination by Macondo oil. Three trends emerge from analysis of the biodegradation rates of 125 individual hydrocarbons in these samples. First, molecular structure served to modulate biodegradation in a predictable fashion, with the simplest structures subject to fastest loss, indicating that biodegradation in the deep ocean progresses similarly to other environments. Second, for many alkanes and polycyclic aromatic hydrocarbons biodegradation occurred in two distinct phases, consistent with rapid loss while oil particles remained suspended followed by slow loss after deposition to the seafloor. Third, the extent of biodegradation for any given sample was influenced by the hydrocarbon content, leading to substantially greater hydrocarbon persistence among the more highly contaminated samples. In addition, under some conditions we find strong evidence for extensive degradation of numerous petroleum biomarkers, notably including the native internal standard 17α(H),21β(H)-hopane, commonly used to calculate the extent of oil weathering.

Testing monsoonal controls on bedrock river incision in the Himalaya and Eastern Tibet with a stochastic‐threshold stream power model

^(10)Be-derived catchment average erosion rates from the Himalaya and Eastern Tibet show different relationships with normalized channel steepness index (k_(sn)), suggesting differences in erosional efficiency of bedrock river incision. We used a threshold stream power model (SPM) combined with a stochastic distribution of discharges to explore the extent to which this observation can be explained by differences in the mean and variability of discharge between the two regions. Based on the analysis of 199 daily discharge records (record lengths 3–45 years; average 18.5 years), we parameterized monsoonal discharge with a weighted sum of two inverse gamma distributions. During both high- and low-flow conditions, annual and interannual discharge variabilities are similarly low in each region. Channel widths for 36 rivers indicate, on average, 25% wider streams in Eastern Tibet than in the Himalaya. Because most catchments with ^(10)Be data are not gauged, we constrained mean annual discharge in these catchments using gridded precipitation data sets that we calibrated to the available discharge records. Comparing ^(10)Be-derived with modeled erosion rates, the stochastic-threshold SPM explains regional differences better than a simple SPM based on drainage area or mean annual runoff. Systematic differences at small k_(sn) values can be reconciled with k_(sn)-dependent erosion thresholds, whereas substantial scatter for high k_(sn) values persists, likely due to methodological limitations. Sensitivity analysis of the stochastic-threshold SPM calibrated to the Himalaya indicates that changes in the duration or strength of summer monsoon precipitation have the largest effect on erosional efficiency, while changes in monsoonal discharge variability have almost no effect. The modeling approach presented in this study can in principle be used to assess the impact of precipitation changes on erosion.

Mio-Pliocene aridity in the south-central Andes associated with Southern Hemisphere cold periods

Significance This paper identifies two periods of enhanced aridity that are synchronous with faunal turnovers in southern South America. Close temporal coincidence with marine climate proxies suggests a period of latest Miocene aridity was associated with a global glacial period and the expansion of C4 vegetation. We thus argue that continental aridity in the south-central Andes is associated with cold periods at high southern latitudes and propose a model to link global and regional (continental) climate via shifting of the Southern Hemisphere westerlies. This paper also provides an example of how 10Be paleo-erosion rates can be used as a climate proxy and demonstrates how 10Be and 36Cl can be combined to reduce uncertainties associated with this method. Although Earth’s climate history is best known through marine records, the corresponding continental climatic conditions drive the evolution of terrestrial life. Continental conditions during the latest Miocene are of particular interest because global faunal turnover is roughly synchronous with a period of global glaciation from ∼6.2–5.5 Ma and with the Messinian Salinity Crisis from ∼6.0–5.3 Ma. Despite the climatic and ecological significance of this period, the continental climatic conditions associated with it remain unclear. We address this question using erosion rates of ancient watersheds to constrain Mio-Pliocene climatic conditions in the south-central Andes near 30° S. Our results show two slowdowns in erosion rate, one from ∼6.1–5.2 Ma and another from 3.6 to 3.3 Ma, which we attribute to periods of continental aridity. This view is supported by synchrony with other regional proxies for aridity and with the timing of glacial ‟cold” periods as recorded by marine proxies, such as the M2 isotope excursion. We thus conclude that aridity in the south-central Andes is associated with cold periods at high southern latitudes, perhaps due to a northward migration of the Southern Hemisphere westerlies, which disrupted the South American Low Level Jet that delivers moisture to southeastern South America. Colder glacial periods, and possibly associated reductions in atmospheric CO2, thus seem to be an important driver of Mio-Pliocene ecological transitions in the central Andes. Finally, this study demonstrates that paleo-erosion rates can be a powerful proxy for ancient continental climates that lie beyond the reach of most lacustrine and glacial archives.

Chronology of tectonic, geomorphic, and volcanic interactions and the tempo of fault slip near Little Lake, California

New geochronologic and geomorphic constraints on the Little Lake fault in the Eastern California shear zone reveal steady, modest rates of dextral slip during and since the mid-to-late Pleistocene. We focus on a suite of offset fluvial landforms in the Pleistocene Owens River channel that formed in response to periodic interaction with nearby basalt flows, thereby recording displacement over multiple time intervals. Overlap between 40 Ar/ 39 Ar ages for the youngest intracanyon basalt flow and 10 Be surface exposure dating of downstream terrace surfaces suggests widespread channel incision during a prominent outburst flood through the Little Lake channel at ca. 64 ka. Older basalt flows flanking the upper and lower canyon margins indicate localization of the Owens River in its current position between 212 ± 14 and 197 ± 11 ka. Coupled with terrestrial light detection and ranging (lidar) and digital topographic measurements of dextral offset, the revised Little Lake chronology indicates average dextral slip rates of at least ∼0.6–0.7 mm/yr and 4 to 10 5 yr. Despite previous geodetic observations of relatively rapid interseismic strain along the Little Lake fault, we find no evidence for sustained temporal fluctuations in slip rates over multiple earthquake cycles. Instead, our results indicate that accelerated fault loading may be transient over much shorter periods (∼10 1 yr) and perhaps indicative of time-dependent seismic hazard associated with Eastern California shear zone faults.

Refining the Southern Extent of the 1872 Owens Valley Earthquake Rupture through Paleoseismic Investigations in the Haiwee Area, Southeastern California

Recent upward revision of the 1872 Owens Valley earthquake from Mw 7.4-7.5 to 7.7-7.9 implies either additional unrecognized rupture length or anoma- louslystronggroundmotionsassociatedwiththisevent.Weinvestigatethefirstpossibility through paleoseismic trenching south of the mapped surface rupture in the Haiwee area, where historical accounts suggest significant surface deformation following the earth- quake. Trenching focused on a prominent north-striking scarp, herein termed the Sage Flat fault, expressed in Pleistocene alluvial fans east of Haiwee Reservoir. Surficial map- ping and ground-based Light Detection and Ranging (lidar) surveying suggest that this faultaccommodateseast-downnormalmotion,andpossiblyacomparableamountofdex- tralslip.Trenchingandluminescencedatingbracketsthetimingofthemostrecentsurface- rupturing earthquake between ∼25:7 and 30.1 ka, and provides evidence for an earlier event predating this time. In combination with scarp profiling, these dates also suggest am aximum rate of normal, dip-slip fault motion up to∼0:1 mm=yr over this period. Although we discovered no evidence for recent surface rupture on the Sage Flat fault, a series of subvertical fractures and fissures cut across young trench stratigraphy, consis- tentwithsecondarydeformationassociatedwithseismicshaking.Assuch,wesuggestthat possible ground disturbance in the Haiwee area during the 1872 event primarily reflected ground shaking or liquefaction-related deformation rather than triggered slip. In addition, we infer a structural and kinematic connection between the Owens Valley fault and oblique-dextral faults north of Lower Cactus Flat in the northwestern Coso Range, rather thanawest-stepintonorthernorwesternRoseValley.Considerationofthesestructuresin the total extent of the Owens Valley fault suggests a length of 140 km, of which at least 113 km ruptured during the 1872 event. Online Material: Procedural details and expanded results from the OSL sample analyses, as well as high-resolution paleoseismic trench logs.