Simon E. Engelhart

Spatial variability of late Holocene and 20th century sea-level rise along the Atlantic coast of the United States

Accurate estimates of global sea-level rise in the pre-satellite era provide a context for 21 st century sea-level predictions, but the use of tide-gauge records is complicated by the contributions from changes in land level due to glacial isostatic adjustment (GIA). We have constructed a rigorous quality-controlled database of late Holocene sea-level indices from the U.S. Atlantic coast, exhibiting subsidence rates of <0.8 mm a –1 in Maine, increasing to rates of 1.7 mm a –1 in Delaware, and a return to rates <0.9 mm a –1 in the Carolinas. This pattern can be attributed to ongoing GIA due to the demise of the Laurentide Ice Sheet. Our data allow us to defi ne the geometry of the associated collapsing proglacial forebulge with a level of resolution unmatched by any other currently available method. The corresponding rates of relative sea-level rise serve as background rates on which future sea-level rise must be superimposed. We further employ the geological data to remove the GIA component from tide-gauge records to estimate a mean 20 th century sea-level rise rate for the U.S. Atlantic coast of 1.8 ± 0.2 mm a –1 , similar to the global average. However, we fia distinct spatial trend in the rate of 20 th century sea-level rise, increasing from Maine to South Carolina. This is the fi rst evidence of this phenomenon from observational data alone. We suggest this may be related to the melting of the Greenland ice sheet and/or ocean steric effects.

Holocene relative sea-level changes and glacial isostatic adjustment of the U.S. Atlantic coast

The first quality-controlled Holocene sea-level database for the U.S. Atlantic coast has been constructed from 686 sea-level indicators. The database documents a decreasing rate of relative sea-level (RSL) rise through time with no evidence of sea level being above present in the middle to late Holocene. The highest rates of RSL rise are found in the mid-Atlantic region. We employ the database to constrain an ensemble of glacial isostatic adjustment models using two ice (ICE-5G, ICE-6G [global ice sheet reconstructions]) and two mantle viscosity (models VM5a,VM5b [VM—radial variation of viscosity in the sublithospheric mantle]) variations to assess whether the spherically symmetric viscoelastic models are able to survive intercomparison with a more refined database of postglacial RSL history. We identify significant misfits between observations and predictions using ICE-5G with the VM5a viscosity profile. ICE-6G provides some improvement for the northern Atlantic region, but misfits remain elsewhere. Decreasing the upper mantle and transition zone viscosity by a factor of 2 to 0.25 × 10 21 Pa s (VM5b) removes significant discrepancies between observations and predictions along the mid-Atlantic coastline, although misfits remain in the southern Atlantic region. These may be an indication of the importance of laterally heterogeneous viscosity in the upper mantle.

Uplift and subsidence reveal a nonpersistent megathrust rupture boundary (Sitkinak Island, Alaska)

We report stratigraphic evidence of land-level change and tsunami inundation along the Alaska-Aleutian megathrust during prehistoric and historical earthquakes west of Kodiak Island. On Sitkinak Island, cores and tidal outcrops fringing a lagoon reveal five sharp lithologic contacts that record coseismic land-level change. Radiocarbon dates, 137Cs profiles, computerized tomography scans, and microfossil assemblages are consistent with rapid uplift circa 290–0, 520–300, and 1050–790 cal yr B.P. and subsidence in A.D. 1964 and circa 640–510 cal yr B.P. Radiocarbon, 137Cs, and 210Pb ages bracketing a sand bed traced 1.5 km inland and evidence for sudden uplift are consistent with Russian accounts of an earthquake and tsunami in A.D. 1788. The mixed uplift and subsidence record suggests that Sitkinak Island sits above a nonpersistent boundary near the southwestern limit of the A.D. 1964 Mw 9.2 megathrust rupture.

Testing the use of microfossils to reconstruct great earthquakes at Cascadia

Coastal stratigraphy from the Pacific Northwest of the United States contains evidence of sudden subsidence during ruptures of the Cascadia subduction zone. Transfer functions (empirical relationships between assemblages and elevation) can convert microfossil data into coastal subsidence estimates. Coseismic deformation models use the subsidence values to constrain earthquake magnitudes. To test the response of foraminifera, the accuracy of the transfer function method, and the presence of a pre-seismic signal, we simulated a great earthquake near Coos Bay, Oregon, by transplanting a bed of modern high salt-marsh sediment into the tidal flat, an elevation change that mimics a coseismic subsidence of 0.64 m. The transplanted bed was quickly buried by mud; after 12 mo and 5 yr, we sampled it for foraminifera. Reconstruction of the simulated coseismic subsidence using our transfer function was 0.61 m, nearly identical to the actual elevation change. Our transplant experiment, and additional analyses spanning the A.D. 1700 earthquake contact at the nearby Coquille River 15 km to the south, show that sediment mixing may explain assemblage changes previously interpreted as evidence of pre-seismic land-level change in Cascadia and elsewhere.

Unusually large tsunamis frequent a currently creeping part of the Aleutian megathrust

Current models used to assess earthquake and tsunami hazards are inadequate where creep dominates a subduction megathrust. Here we report geological evidence for large tsunamis, occurring on average every 300–340 years, near the source areas of the 1946 and 1957 Aleutian tsunamis. These areas bookend a postulated seismic gap over 200 km long where modern geodetic measurements indicate that the megathrust is currently creeping. At Sedanka Island, evidence for large tsunamis includes six sand sheets that blanket a lowland facing the Pacific Ocean, rise to 15 m above mean sea level, contain marine diatoms, cap terraces, adjoin evidence for scour, and date from the past 1700 years. The youngest sheet and modern drift logs found as far as 800 m inland and >18 m elevation likely record the 1957 tsunami. Previously unrecognized tsunami sources coexist with a presently creeping megathrust along this part of the Aleutian Subduction Zone.

Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska

Despite the role of the Alaska-Aleutian megathrust as the source of some of the largest earthquakes and tsunamis, the history of its pre–twentieth century tsunamis is largely unknown west of the rupture zone of the great (magnitude, M 9.2) 1964 earthquake. Stratigraphy in core transects at two boggy lowland sites on Chirikof Island’s southwest coast preserves tsunami deposits dating from the postglacial to the twentieth century. In a 500-m-long basin 13–15 m above sea level and 400 m from the sea, 4 of 10 sandy to silty beds in a 3–5-m-thick sequence of freshwater peat were probably deposited by tsunamis. The freshwater peat sequence beneath a gently sloping alluvial fan 2 km to the east, 5–15 m above sea level and 550 m from the sea, contains 20 sandy to silty beds deposited since 3.5 ka; at least 13 were probably deposited by tsunamis. Although most of the sandy beds have consistent thicknesses (over distances of 10–265 m), sharp lower contacts, good sorting, and/or upward fining typical of tsunami deposits, the beds contain abundant freshwater diatoms, very few brackish-water diatoms, and no marine diatoms. Apparently, tsunamis traveling inland over low dunes and boggy lowland entrained largely freshwater diatoms. Abundant fragmented diatoms, and lake species in some sandy beds not found in host peat, were probably transported by tsunamis to elevations of >10 m at the eastern site. Single-aliquot regeneration optically stimulated luminescence dating of the third youngest bed is consistent with its having been deposited by the tsunami recorded at Russian hunting outposts in 1788, and with the second youngest bed being deposited by a tsunami during an upper plate earthquake in 1880. We infer from stratigraphy, 14C-dated peat deposition rates, and unpublished analyses of the island’s history that the 1938 tsunami may locally have reached an elevation of >10 m. As this is the first record of Aleutian tsunamis extending throughout the Holocene, we cannot estimate source earthquake locations or magnitudes for most tsunami-deposited beds. We infer that no more than 3 of the 23 possible tsunamis beds at both sites were deposited following upper plate faulting or submarine landslides independent of megathrust earthquakes. If so, the Semidi segment of the Alaska-Aleutian megathrust near Chirikof Island probably sent high tsunamis southward every 180–270 yr for at least the past 3500 yr.

Subsidence along the Atlantic Coast of North America: Insights from GPS and late Holocene relative sea level data

The Atlantic Coast of North America is increasingly affected by flooding associated with tropical and extratropical storms, exacerbated by the combined effects of accelerated sea-level rise and land subsidence. The region includes the collapsing forebulge of the Laurentide Ice Sheet. High-quality records of late Holocene relative sea-level (RSL) rise are now available, allowing separation of long-term glacial isostatic adjustment-induced displacement from modern vertical displacement measured by GPS. We compare geological records of late Holocene RSL to present-day vertical rates from GPS. For many coastal areas there is no significant difference between these independent data. Exceptions occur in areas of recent excessive groundwater extraction, between Virginia (38°N) and South Carolina (32.5°N). The present-day subsidence rates in these areas are approximately double the long-term geologic rates, which has important implications for flood mitigation. Tide gauge records, therefore, should be used with caution for studying sea-level rise in this region.

The contribution of glacial isostatic adjustment to projections of sea-level change along the Atlantic and Gulf coasts of North America

We determine the contribution of glacial isostatic adjustment (GIA) to future relative sea-level change for the North American coastline between Newfoundland and Texas. We infer GIA model parameters using recently compiled and quality-assessed databases of past sea-level changes, including new databases for the United States Gulf Coast and Atlantic Canada. At 13 cities along this coastline, we estimate the GIA contribution to range from a few centimeters (e.g., 3 [−1 to 9] cm Miami) to a few decimeters (e.g., 18 [12–22] cm, Halifax) for the period 2085–2100 relative to 2006–2015 (1−σ ranges given). We provide estimates of uncertainty in the GIA component using two different methods; the more conservative approach produces total ranges (1−σ confidence) that vary from 3 to 16 cm for the cities considered. Contributions from ocean steric and dynamic changes as well as those from changes in land ice are also estimated to provide context for the GIA projections. When summing the contributions from all three processes at the 13 cities considered along this coastline, using median or best-estimate values, the GIA signal comprises 5–38% of the total depending on the adopted climate forcing and location. The contributions from ocean dynamic/steric changes and ice mass loss are similar in amplitude but with spatial variation that approximately cancels, resulting in GIA dominating the net spatial variability north of 35°N.

Stratigraphic and microfossil evidence for a 4500-year history of Cascadia subduction zone earthquakes and tsunamis at Yaquina River estuary, Oregon, USA

We infer that each buried soil represents a Cascadia subduction zone earthquake because the soils are laterally extensive and abruptly overlain by sandy deposits and mud. Preservation of coseismically buried soils occurred from 4500 yr ago until ~500–600 yr ago, after which preservation was compromised by cessation of gradual relative sea-level rise, which in turn precluded drowning of marsh soils during instances of coseismic subsidence. Based on grain-size and microfossil data, sandy deposits overlying buried soils accumulated immediately after a subduction zone earthquake, during tsunami incursion into Sallys Bend. The possibility that the sandy deposits were sourced directly from landslides triggered upstream in the Yaquina River basin by seismic shaking was discounted based on sedimentologic, microfossil, and depositional site characteristics of the sandy deposits, which were inconsistent with a fluvial origin. Biostratigraphic analyses of sediment above two buried soils—in the case of two earthquakes, one occurring shortly before 1541–1708 cal. yr B.P. and the other occurring shortly before 3227–3444 cal. yr B.P.—provide estimates that coseismic subsidence was a minimum of 0.4 m. The average recurrence interval of subduction zone earthquakes is 420–580 yr, based on an ~3750–4050-yr-long record and seven to nine interearthquake intervals.

Accommodation space, relative sea level, and the archiving of paleo-earthquakes along subduction zones

The spatial variability of Holocene relative sea-level (RSL) change influences the capacities of coastal environments to accommodate a sedimentary record of paleoenvironmental change. In this study we couch a specific investigation in more general terms in order to demonstrate the applicability of the relative sea-level history approach to paleoseismic investigations. Using subsidence stratigraphy, we trace the different modes of coastal sedimentation over the course of time in the eastern Indian Ocean where RSL change evolved from rapidly rising to static from 8000 yr ago to present. Initially, the coastal sites from the Aceh, Sumatra, coastal plain, which are subject to repeated great earthquakes and tsunamis, built up a sedimentary sequence in response to a RSL rise of 1.4 mm/yr. The sequence found at 2 sites 8 km apart contained 3 soils of a mangrove origin (Rhizophora, Bruguiera/Ceriops, Avicennia pollen, and/or intertidal foraminifera) buried by sudden submergence related to coseismic subsidence and 6 tsunami sands that contain pristine subtidal and planktic foraminifera. After 3800 cal yr B.P. (years before A.D. 1950), sea level stabilized and remained such to the present. The stable relative sea level reduced accommodation space in the late Holocene, suggesting that the continued aggradation of the coastal plain was a consequence of periodic coastal inundation by tsunamis.