Papers by Dorothy Merritts
AGU Fall Meeting Abstracts, Dec 1, 2019
Earth and Planetary Science Letters, Nov 1, 2018
The Mendocino Triple Junction (MTJ) region in northern California is an archetype for studying la... more The Mendocino Triple Junction (MTJ) region in northern California is an archetype for studying landscape response to varying rock uplift rates as they increase toward the triple junction. Underpinning these studies lies the assumption that this landscape has reached a dynamic equilibrium in which rock uplift and erosion rates are equal; however, no study has shown that such an equilibrium actually exists around the MTJ region. Here, we report 10 Be-and 26 Al-derived erosion rates calculated from isotope concentrations in detrital, fluvial sediment from coastal drainage basins; we then compare these rates with uplift rates inferred from marine terraces that were formed and preserved by uplift during the last ∼305 ka. Erosion rates in the more slowly uplifting southern part of the region range from 0.21-0.32 mm/yr and are consistent with rock uplift rates since 305 ka. However, in the northern transition zone, where uplift rates apparently started to increase about ∼100 ka due to northward migration of the MTJ, erosion rates are higher, 0.43 to 0.69 mm/yr. These rates are similar to uplift rates from 96-305 ka, but substantially less than recent uplift rates of ∼3.5-4 mm/yr inferred for the past ∼72 ka. In the central part of the King Range, finite erosion rates cannot be determined due to (i) the presence of excess 10 Be that does not appear to have originated from in-situ cosmic-ray production during erosion and transport of the sediment, and (ii) 26 Al concentrations below detection limits. The difference between catchment-wide erosion and recent uplift rates in coastal catchments may be related to a lag in response of hillslopes and tributaries to changes in rates of tectonically-driven relative baselevel fall or the uncertainties in rock uplift rates inferred from marine terraces. This study suggests that measurements of both erosion and uplift rates are crucial in assessing the equilibrium state of landscapes and in understanding the topographic features made by surface and tectonic processes.
AGU Fall Meeting Abstracts, Dec 1, 2020
AGU Fall Meeting Abstracts, Dec 1, 2018
EGU General Assembly Conference Abstracts, Apr 1, 2018
Abstracts with programs, 2018
Abstracts with programs, 2018
51st Annual Northeastern GSA Section Meeting, 2016
AGU Fall Meeting 2020, Dec 10, 2020
AGU Fall Meeting Abstracts, Dec 18, 2015
Fluvial terraces have been studied and dated along tectonically active and inactive plate margins... more Fluvial terraces have been studied and dated along tectonically active and inactive plate margins at multiple sites around the world, revealing that bedrock incision and strath terrace formations keep pace with bedrock uplift, leading to graded long-stream profiles over tens of thousands of years. Climate change associated with glacial–interglacial cycles provokes different responses in nonglacial fluvial systems, with some streams aggrading and others degrading during cold glacial periods. Aggradation is particularly surprising for coastal streams during the Last Glacial Maximum (LGM), when sea level was about 120 m lower than during interglacial warm periods. The role of climate versus sea level in driving rates of river erosion and offshore sediment deposition has been studied from offshore sedimentary records as well, and it is likely that continued studies of terraces will reveal new insights about the links between climate, sea level, river erosion, and offshore sedimentation.
Cambridge University Press eBooks, Jun 1, 2010
AGU Fall Meeting Abstracts, Dec 1, 2020
Abstracts with programs, 2022
Palaeontologia Electronica, 2016
Beginning in the seventeenth century, colonial activities such as land clearing, agriculture, and... more Beginning in the seventeenth century, colonial activities such as land clearing, agriculture, and milldam construction significantly altered the landscapes, vegetation, and hydrogeomorphology of the northeastern Piedmont region, modern-day USA. Presently, weedy and non-native vegetation dominate the altered riparian zones and hill slopes where old-growth, hardwood forests once prevailed. Various studies have identified pre-settlement herbaceous wetland floras based on fruits and seeds. However, only one report provided evidence of the dominant pre-settlement woody species by using the localized signal available from subfossil leaves; that work reconstructed Maple-Ash floodplain swamp forests along valley-margins and Oak-Beech mixed forests on upper-slopes at a site in Lancaster County, southeastern Pennsylvania. Here, we investigate subfossil leaves recovered from the buried wetland soils of White Clay Creek in neighboring Chester County, Pennsylvania, providing significant new spatial data to our understanding of regional old-growth forests. The leaf assemblage, radiocarbon dated to ca. 1650, is composed of woody species with a diverse range of wetland affinities, indicating sources in both the riparian zone and surrounding lower hill slopes. Obligate and facultative-wetland species include willow (Salix spp.) and Hazel Alder (Alnus serrulata). Box Elder (Acer negundo) is the only facultative species present, while American Beech (Fagus grandifolia), the white and red oak groups (Quercus Section Quercus and Q. Section Lobatae, respectively), and Tulip Tree (Liriodendron tulipifera) comprise the identified facultative-upland taxa. These results complement and corroborate previous work and allow for greater confidence in understanding the pre-European landscape, potentially increasing the effectiveness of regional environmental restoration projects.
Eos, Transactions American Geophysical Union, 2000
Geomorphologists have an insiders' joke that I'll share here, as Eos garners a pa... more Geomorphologists have an insiders' joke that I'll share here, as Eos garners a particularly appropriate audience: ``There are as many geomorphologists who think that they're doing geophysics as there are geophysicists who think that they're doing geomorphology, but neither group is doing it very well.''The allegation misrepresents the truth, of course, but it does reflect upon a trend that grew
Annual Review of Earth and Planetary Sciences, May 31, 2022
Just as glaciers worldwide left a record of past advances and retreats that shifted latitudinally... more Just as glaciers worldwide left a record of past advances and retreats that shifted latitudinally in response to oscillating Quaternary climate changes, so too have cold-climate conditions and permafrost left topographic and sedimentary signatures in former periglacial environments. This review documents widespread occurrence of past permafrost and intense frost action that led to rock fracturing, regolith production, and regolith-mantled slopes in the mid-Atlantic region of the United States during late Pleistocene cold-climate conditions. Strong signatures of thermal contraction cracking and brecciation from frost cracking exist where rocks and sediments are most frost susceptible, as with fissile shales. On sandstone hillslopes, frost weathering produced boulder-rich sediment that episodically flowed slowly down-slope during permafrost thaw, resulting in solifluction lobes and terraces in which colluvium moved cumulatively at least a kilometer. Radiocarbon dating, optically stimulated luminescence age control, and cosmogenic isotope studies constrain some periglacial features to the Last Glacial Maximum but also indicate longer residence times of regolith. ▪ Former permafrost and areas of intensive frost cracking extended over much of the mid-Atlantic region of the eastern United States during late Pleistocene cold glacial periods. ▪ Cold-climate conditions and permafrost left long-lasting topographic and sedimentary records with limited post-depositional erosion in the formerly periglacial mid-Atlantic region. ▪ Prominent relict periglacial landforms include polygon networks and frost wedges that are the result of thermal contraction cracking and brecciated rock formed by segregated ice and frost cracking. ▪ Widespread solifluction landforms are a topographic signature of freezing, thawing, and mass movement of mobile regolith produced by frost cracking, and some were active during the Last Glacial Maximum.
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Papers by Dorothy Merritts