Nicholas Pinter

Quaternary uplift astride the aseismic Cocos Ridge, Pacific coast, Costa Rica.

The Pacific coast of Costa Rica lies within the Central American forearc and magmatic-arc region that was created by northeastward subduction of the Cocos plate beneath the Caribbean plate at the Middle America Trench. From the Peninsula de Nicoya south-eastward toward the Peninsula de Osa and the Peninsula de Burica on the Panamanian border, the Middle America Trench loses its physiographic expression where it intersects the aseismic Cocos Ridge. Interaction between subduction of the buoyant, aseismic Cocos Ridge and the overriding Caribbean plate is invoked to explain the variation in rates of vertical crustal uplift along a coastal transect from Nicoya to Burica. The Pliocene and Pleistocene stratigraphic record and Holocene marine terraces and beach ridge complexes indicate that maximum rates of crustal uplift have occurred on the Peninsula de Osa, immediately landward of the aseismic Cocos Ridge. Crustal uplift rates decrease northwest toward the Peninsula de Nicoya, and to a lesser extent southwest toward the Peninsula de Burica. The late Quaternary stratigraphy on the Peninsula de Osa is subdivided into two major chronostratigraphic sequences from groupings of radiocarbon dates. Crustal uplift rates calculated from these sequences systematically decrease from 6.5 to 2.1 m/ka north-east across the peninsula. Deformation of the peninsula is modeled as uplifted and down-to-the-northeast-tilted fault blocks with an angular rotation rate of 0.03° to 0.06° per thousand years. Although less well constrained, crustal uplift rates on the Peninsula de Nicoya, 200 km to the northwest of the Peninsula de Osa, vary from <1 m/ka for Pliocene and Pleistocene sediments to 2.5 m/ka for Holocene marine terraces. In the Quepos region, 100 km to the northwest of the Peninsula de Osa, calculated uplift rates derived from incision of late Quaternary fluvial terraces range from 0.5 to 3.0 m/ka. On the Peninsula de Burica, only 60 km to the southwest of the Peninsula de Osa, calculated uplift rates range from 4.7 m/ka for a late Holocene marine terrace to 1.2 m/ka for post-late Pliocene deep-sea sediments. The variations in calculated uplift rates on the Peninsula de Osa constrain a dynamic model for subduction of the Cocos Ridge and the resulting uplift of the overriding Caribbean plate. Deflection of the Caribbean plate is modeled using various effective elastic thicknesses as the response of an elastic plate to the buoyant force of the subducted Cocos Ridge. Because the shape of the subducted end of the Cocos Ridge is unknown, two scenarios are evaluated: (1) a radially symmetric ridge with a slope similar to the slope of the flanks of the ridge and (2) a ridge where the subducted end was truncated by the Panama fracture zone. The best-fit model utilizes a truncated ridge that has been subducted during the past 0.5 m.y. ∼50 km beneath the overriding Caribbean plate, which has an effective elastic thickness of 5 km. The model predicts that the highest uplift rate should be ∼3.7 m/ka and occur on the southwest coast of the Peninsula de Osa. The rate of uplift slows considerably to the northeast and indicates that the Peninsula de Osa is tilting to the northeast, which agrees with observations in that region. The predicted uplift rate attributed to aseismic ridge subduction also decreases along the coast both north and south of the Peninsula de Osa, resulting in little uplift that can be attributed to Cocos Ridge subduction in the northwestern portions of the Peninsula de Nicoya.

Impacts, mega-tsunami, and other extraordinary claims

We welcome the comments of Abbott et al., Firestone and West, and Bunch et al.; in fact, we anticipated and hoped for the opportunity to further discuss the rash of recent impact claims and alternative interpretations. Abbott et al. reiterate claims for multiple large Holocene ocean impacts, resulting in “mega-tsunami” recorded by “chevrons.” Longterm meteorological records from stations near two chevron sites show the features to be precisely aligned with dominant winds (Pinter and Ishman, 2008), and their morphology is that of parabolic dunes. Abbott et al. argue that it is impossible to transport marine microfossils into these chevrons by wind. In actuality, coastal dune sand is commonly full of marine micro(and macro) fossils. Two samples of Pleistocene eolianite that we collected in California from 114 m and 230 m elevation contained numerous well-preserved foraminifera spanning 11 genera. Abbott et al. also argue that ironand chromium-stained foraminifera in chevrons and ocean cores represent splashes of impact melt. CaCO 3 melts are documented for the Chicxulub impact site, but these are a far cry from “well-preserved carbonate microfossils” encased in silicate melt. Salge (2007) describes silica and associated carbonate at Chicxulub as “carbonate melt particles,” recrystallized and decomposed calcite, and CO 2 back-reacting to calcite. Abbott et al. also state “[r]ecent experiments have replicated grasses intact within silicate impact melts.” However, the Harris and Schultz (2007) abstract merely suggests a mechanism for “grass-like remains in impact melt breccias.” Fully carbonized plant remains are documented in the geological record (e.g., Scott, 2008), but few researchers would accept unaltered plant material or foraminiferal tests in direct contact with silicate or metallic impact melt. The comments by Firestone and West and Bunch et al. reiterate the claim for a catastrophic North American impact event at 12.9 ka, as presented in Firestone et al. (2006, 2007a). Firestone and West try to distance themselves from earlier claims (e.g., “cosmicray jets,” “deadly nerve toxins” in Pleistocene algal mats, etc.), yet Firestone et al. (2007b) identify a new 33-ka airburst impact event (i.e., no crater) based on micrometeorite “bullets” lodged in mammoth tusks. Regarding the “12.9 ka event,” Firestone et al. (2007a) present an unusual assortment of evidence well outside criteria accepted by the impact community (e.g., Reimold, 2007). A common theme among the bevy of marginal impact claims is that they eschew accepted criteria in favor of new and untested markers. Our article offers an alternative hypothesis. Bunch et al. advocate that spikes in microspherule frequency require an impact event— we suggest that much of this material represents micrometeorite ablation fallout, which can and should be concentrated at any depositional hiatus or in any condensed section (see calculations at www.geology.siu.edu/GSATSupplement.pdf). Further concentration could result from sediment reworking or from terrestrial sources. It is suggestive that Firestone et al.’s highest spherule counts are from Michigan and Alberta, downwind of the ca. 13 ka eruption sequence at Glacier Peak, Washington (Mastin and Waitt, 2000). Bunch et al. and Firestone and West also highlight iridium at up to 117 ppb, higher than many Cretaceous-Tertiary sites. This comparison is disingenuous. The 117 ppb concentration and another of 51 ppb Ir were from spherule and magnetic grain separates, fully consistent with a noncatastrophic micrometeorite source (see www.geology.siu.edu/GSATSupplement.pdf). Firestone et al.’s bulk Ir concentrations peaked at just 2.3–3.8 ppb; maximum values were below detection at six of their 10 sites. Firestone et al. (2007a) note that these values are anomalously low, requiring them to invoke an Ir-depleted impactor. Our hypothesis of ubiquitous micrometeorite debris, concentrated by variation in clastic input and other terrestrial mechanisms, is eminently testable. We predict elevated concentrations of most or all of Firestone et al.’s markers at multiple time horizons in similar sequences. Our research group and several others are now separating microspherules collected from a range of sites and ages. The 12.9 ka impact hypothesis may be confirmed, or it may soon run into a wall of contrary data. The latest Pleistocene and early Holocene span a period of dramatic paleo-environmental changes. Against such a background, workers seeking grand unifying explanations should be aware of the danger of “selection bias.” Selection bias was invoked following claims for Paleolithic artifacts at the Calico site in California. At Calico, ~200 crude man-made objects were identified among countless fractured and unfractured clasts in a chert-rich fanglomerate. After years of acrimonious debate, scientific consensus recognized that the presumed artifacts represented “a biased sample of lithics from the total population of naturally fractured lithics at that site” (Duvall and Venner, 1979)—i.e., a few markers collected in good faith from an abundant background, combined with a good story and some wishful thinking. The real lesson of Calico may be that 30+ years later, a few hardcore proponents cling to the original story. We have no desire to squelch new scientific hypotheses, but the danger is these “extraordinary claims,” once disseminated in the popular imagination, can be almost impossible to dispel.

Assessing flood hazard on dynamic rivers

Recurrent flooding of the Red River in 1997, North Carolinas Tar River in 1999, and the Mississippi River in 1993, 1995, and 2001 has led to speculation that human activities contribute significantly to these disasters. New analyses of historical hydrologic data from the Mississippi River quantify systematic increases in flood stages for equal discharges. Analyses of cross-sectional measurements and other evidence suggest that rising stages result from flow retardation by navigational dikes and floodwater confinement by levees. On dynamic rivers, flood-frequency statistics must be revised periodically for hazard assessments to keep pace with current conditions, but the present reassessment methodology is prohibitively time-consuming and expensive.

No evidence of nanodiamonds in Younger–Dryas sediments to support an impact event

The causes of the late Pleistocene megafaunal extinctions in North America, disappearance of Clovis paleoindian lithic technology, and abrupt Younger–Dryas (YD) climate reversal of the last deglacial warming in the Northern Hemisphere remain an enigma. A controversial hypothesis proposes that one or more cometary airbursts/impacts barraged North America ≈12,900 cal yr B.P. and caused these events. Most evidence supporting this hypothesis has been discredited except for reports of nanodiamonds (including the rare hexagonal polytype) in Bølling–Ållerod-YD-boundary sediments. The hexagonal polytype of diamond, lonsdaleite, is of particular interest because it is often associated with shock pressures related to impacts where it has been found to occur naturally. Unfortunately, previous reports of YD-boundary nanodiamonds have left many unanswered questions regarding the nature and occurrence of the nanodiamonds. Therefore, we examined carbon-rich materials isolated from sediments dated 15,818 cal yr B.P. to present (including the Bølling–Ållerod-YD boundary). No nanodiamonds were found in our study. Instead, graphene- and graphene/graphane-oxide aggregates are ubiquitous in all specimens examined. We demonstrate that previous studies misidentified graphene/graphane-oxide aggregates as hexagonal diamond and likely misidentified graphene as cubic diamond. Our results cast doubt upon one of the last widely discussed pieces of evidence supporting the YD impact hypothesis.

Late Quaternary slip on the Santa Cruz Island fault, California

The style, timing, and pattern of slip on the Santa Cruz Island fault were investigated by trenching the fault and by analysis of offset late Quaternary landforms. A trench excavated across the fault at Christi Beach, on the western coast of the island, exposed deformation of latest Pleistocene to Holocene sediments and pre-Quaternary rocks, recording repeated large-magnitude rupture events. The most recent earthquake at this site occurred ca. 5 ka. Coastal terraces preserved on western Santa Cruz Island have been dated using the uranium-series technique and by extrapolation using terrace elevations and the eustatic record. Offset of terraces and other landforms indicates that the Santa Cruz Island fault is predominantly left lateral, having a horizontal slip rate of not more than 1.1 mm/yr and probably about 0.8 mm/yr. The fault also has a smaller reverse component, slipping at a rate of between 0.1 and 0.2 mm/yr. Combined with measurements of slip per event, this information suggests a long-term average recurrence interval of at least 2.7 k.y. and probably 4–5 k.y., and average earthquake magnitudes of Mw 7.2–7.5. Sense of slip, recurrence interval, and earthquake magnitudes calculated here for the Santa Cruz Island fault are very similar to recent results for other faults along the southern margin of the western Transverse Range, including the Malibu Coast fault, the Santa Monica fault, the Hollywood fault, and the Raymond fault, supporting the contention that these faults constitute a continuous and linked fault system, which is characterized by large but relatively infrequent earthquakes.

Geomorphological analysis of neotectonic deformation, northern Owens Valley, California

At the western edge of the Basin and Range Province, the Owens Valley is the site of active seismicity and deformation. Morphometric analyses of three geomorphological features are used to determine the patterns and rates of neotectonic deformation: (l) a network of Pleistocene channels cut on top of the Bishop Tuff; (2) uplifted terraces of the Owens River; and (3) alluvial fans of the White Mountain front.In the Owens Valley, the three analyses are consistent with the same solution: net eastward tilt of the Owens Valley block at a rate of between 3.5 and 6.1°/Ma. Given the dip on the basement determined from geophysical data and extrapolating the rate of tilt in the Owens Valley back in time, it is inferred that the break-up of the Sierra Nevada and the northern Owens Valley occurred in the Pliocene, between around 2 and 4 Ma ago. The pattern of deformation in the northern Owens Valley matches anticlinal flexure on the Coyote warp, near the front of the Sierra Nevada, and faulting across the Volcanic Tableland is consistent with flexural extension. It is proposed that the Coyote warp is an expression of the tectonic hinge between westward rotation of the Sierra Nevada and eastward rotation of the Owens Valley since the Pliocene.

Construction of a polynomial model of glacio-eustatic fluctuation: Estimating paleo-sea levels continuously through time

Sea-level curves for the late Quaternary are typically reconstructed with data from reef tracts and terraces preserved along actively uplifting coastlines, recording highstands, and in some cases lowstands, of the world9s oceans through time. As a result, sea-level curves reconstructed from these data are fixed at the maxima (highstands) and minima (lowstands), but either make no predictions about the intervening time or use arbitrary or free-hand methods of interpolation. These curves fail to meet the need of many applications of sea-level data for continuous, quantitative information. A new method for calculating sea levels during intervening times produces a polynomial expression of sea level through time. LaGrange polynomial interpolation is used to fit a series of fourth-order equations to existing data of late Quaternary sea-level fluctuations, producing a systematic and realistic model of continuous sea-level elevations with time. The method is germane to any and all models of sea level based on discrete data points. Here the model advanced by Bloom et a1. is reexamined, and a new polynomial model which estimates eustatic sea levels continuously in the range 0-140 ka is developed from the Huon data. The method includes an error-analysis procedure that accounts for the effects of error in radiometric dates and theodolite measurements in the original data set, and extrapolates this error through the continuous model. Polynomial form and error estimation are indispensible to rigorous quantitative applications of sea-level data and supplant less-rigorous methods used to approximate sea level and rates of sea-level rise.

Arguments and Evidence Against a Younger Dryas Impact Event

We present arguments and evidence against the hypothesis that a large impact or airburst caused a significant abrupt climate change, extinction event, and termination of the Clovis culture at 12.9 ka. It should be noted that there is not one single Younger Dryas (YD) impact hypothesis but several that conflict with one another regarding many significant details. Fragmentation and explosion mechanisms proposed for some of the versions do not conserve energy or momentum, no physics-based model has been presented to support the various concepts, and existing physical models contradict them. In addition, the a priori odds of the impact of a >4 km comet in the prescribed configuration on the Laurentide Ice Sheet during the specified time period are infinitesimal, about one in 10 15 . There are three broad classes of counterarguments. First, evidence for an impact is lacking. No impact craters of the appropriate size and age are known, and no unambiguously shocked material or other features diagnostic of impact have been found in YD sediments. Second, the climatological, paleontological, and archeological events that the YD impact proponents are attempting to explain are not unique, are arguably misinterpreted by the proponents, have large chronological uncertainties, are not necessarily coupled, and do not require an impact. Third, we believe that proponents have misinterpreted some of the evidence used to argue for an impact, and several independent researchers have been unable to reproduce reported results. This is compounded by the observation of contamination in a purported YD sample with modern carbon.

Fault-Related Folding in California's Northern Channel Islands Documented by Rapid-Static GPS Positioning

Rapid-static Global Positioning System positioning was used to measure late Quaternary deformation of uplifted coastal terraces on Santa Rosa and San Miguel Islands, which have accumulated regional warping and local faulting over 100+ k.y. Late Quaternary deformation on Santa Rosa is sharply partitioned, with several hundred meters of surfacerupturing left slip, but 15+ m of vertical motion taken up by folding, resulting in anticlinal growth of at least 0.12 m/k.y. Deformation on San Miguel Island to the west is consistent with warping on the north limb of the regional anticlinal structure. Deformation on both islands matches activity to the east, suggesting that the Santa Rosa Island fault represents an en echelon segment of the larger Transverse Ranges Boundary fault system and the Northern Channel Islands antiform. The Northern Channel Islands have been the focus of a debate over differing geometrical models of fault-related folding, and the deformation measurements presented here suggest that uplift of the islands is occurring over a smoothly curved thrust ramp rather than a fault-bend fold. This reinterpretation is important, as current earthquake-hazard assessments on this and other buried reverse faults depend entirely on assumptions unique to the fault-bend fold geometry. We suggest that alternative models should be considered where geologically reasonable, and that surface uplift measurements across fault-related folds can provide a crucial test of subsurface geometry.

Regional Impacts of Levee Construction and Channelization, Middle Mississippi River, USA

The Mississippi River travels approximately 3780 km from its headwaters in Minnesota, USA to the Gulf of Mexico, draining roughly 3,210,000 km2. Since European settlement, humans have significantly impacted both the river basin and the river itself. Primarily under the stewardship of the U.S. Army Corps of Engineers (USACE), river-engineering activities have focused on navigation improvement and flood control and have included snag removal, bank clearing, meander cutoffs, dredging, bank stabilization, channel constriction, levee construction, and dam construction [1].