Amino Acid Age Estimates Research Articles

Age model for a continuous, ca 250-ka Quaternary lacustrine record from Bear Lake, Utah–Idaho

The Quaternary sediments sampled by continuous 120-m-long drill cores from Bear Lake (Utah-Idaho) comprise one of the longest lacustrine sequences recovered from an extant lake. The cores serve as a good case study for the construction of an age model for sequences that extend beyond the range of radiocarbon dating. From a variety of potential age indicators, we selected a combination of radiocarbon ages, one magnetic excursion (correlated to a standard sequence), and a single Uranium-series age to develop an initial data set. The reliability of the excursion and U-series data require consideration of their position with respect to sediments of inferred interglacial character, but not direct correlation with other paleoclimate records. Data omitted from the age model include amino acid age estimates, which have a large amount of scatter, and tephrochronology correlations, which have relatively large uncertainties. Because the initial data set was restricted to the upper half of the BL00-1 core, we inferred additional ages by direct correlation to the independently dated paleoclimate record from Devils Hole. We developed an age model for the entire core using statistical methods that consider both the uncertainties of the original data and that of the curve-fitting process, with a combination of our initial data set and the climate correlations as control points. This age model represents our best estimate of the chronology of deposition in Bear Lake. Because the age model contains assumptions about the correlation of Bear Lake to other climate records, the model cannot be used to address some paleoclimate questions, such as phase relationships with other areas.

Deglaciation of the Irish Sea Basin: a critique of the glaciomarine hypothesis

The glaciomarine model for deglaciation of the Irish Sea basin suggests that the weight of ice at the last glacial maximum was sufficient to raise relative sea-levels far above their present height, destabilising the ice margin and causing rapid deglaciation. Glacigenic deposits throughout the basin have been interpreted as glaciomarine. The six main lines of evidence on which the hypothesis rests (sedimentology, deformation structures, delta deposits, marine fauna, amino-acid ratios and radiocarbon dates) are reviewed critically. The sedimentological interpretation of many sections has been challenged and it is argued that subglacial sediments are common rather than rare and that there is widespread evidence of glaciotectonism. Density-driven deformation associated with waterlain sediments is rare and occurs where water was ponded locally. Sand and gravel deposits interpreted as Gilbert-type deltas are similarly the result of local ponding or occur where glaciers from different source areas uncoupled. They do not record past sea-levels and the ad hoc theory of ‘piano-key tectonics’ is not required to explain the irregular pattern of altitudes. The cold-water foraminifers interpreted as in situ are regarded as reworked from Irish Sea sediments that accumulated during much of the late Quaternary, when the basin was cold and shallow with reduced salinities. Amino-acid age estimates used in support of the glaciomarine model are regarded as unreliable. Radiocarbon dates from distinctive foraminiferal assemblages in northeast Ireland show that glaciomarine sediments do occur above present sea-level, but they are restricted to low altitudes in the north of the basin and record a rise rather than a fall in sea-level. It is suggested here that the oldest dates, around 17 000 yr BP, record the first Late Devensian (Weichselian) marine inundation above present sea-level. This accords with the pattern but not the detail of recent models of sea-level change. Copyright © 2001 John Wiley & Sons, Ltd.

Amino acid racemization geochronology of reworked Quaternary mollusks on U.S. Atlantic coast beaches: implications for chronostratigraphy, taphonomy, and coastal sediment transport

Approximately 200 shells (primarily Mercenaria) from 21 beach sites between New Jersey and Florida have been analyzed for the extent of racemization (epimerization) of their fossilized amino acids. The greatest concentration of sites is in North Carolina, in regions least affected by human modification of natural beach sediment-transport processes. These results can be used to estimate the frequency of age mixing of Pleistocene shell material in Holocene depositional environments. Selected shells have also been dated by 14C (conventional and/or AMS) to provide direct calibration of the amino acid epimerization ratios. Shell taphonomic characteristics (particularly color) can be qualitatively related to apparent ages. Data for shell fragmentation, combined with amino acid age estimates, provide insights into probable transport distances of reworked shells. Shells with apparent Pleistocene ages have epimerization values equivalent to those seen in onshore exposures of Pleistocene coastal units. Amino acid epimerization measurements are a comparatively rapid and inexpensive chronologic tool for use in taphonomic and sediment(shell-) transport studies. Paired amino acid- 14C analyses on single shells provide insights into geochemical alteration of shells and permit modelling of the residence time of shells on beaches. Greater numbers of analyses at each site are needed before statistically valid estimates of age mixing can be obtained, but the apparent distribution of Pleistocene shells on Atlantic coast beaches appears to be related to the distribution of Pleistocene units in the shoreface and inner shelf, and the thickness of Holocene sedimentary cover in specific areas. These observations provide useful criteria for the evaluation of possible age mixing of shells collected at outcrops of Pleistocene units in the Coastal Plain.

Late middle pleistocene interglacial deposits at upper strensham, worcestershire, england

AbstractPleistocene deposits containing the disarticulated skeleton of a mammoth, and associated faunal and floral remains, were discovered in July 1990 at Upper Strensham, Worcestershire. The environmental evidence from the fauna and flora together with limited geological evidence, indicates that the deposits accumulated within a low energy fluvial environment with a surrounding marsh and restricted tree cover on, or close to, the floodplain. The patchy occurrence of trees in a species‐rich grassland is discussed, and the climatic significance of the fauna and flora is considered.The Strensham site lies within the valley of the River Avon, which is known to contain at least five altitudinally distinct river terraces. The deposits at Strensham lie beneath a terraced surface that cannot be accommodated within the existing framework of terrace development in the valley, and evidence is presented which may suggest that these deposits form a previously unrecognised fluvial unit, the Strensham Member of the Avon Valley Formation. Amino‐acid age estimates from shells taken from the fossiliferous sediments of the Strensham Member suggest a correlation with Oxygen Isotope Stage 7. This correlation suggests that the temperate deposits at this site should be correlated with the temperate phase recorded at Marsworth, Buckinghamshire and Stoke Goldington in the valley of the Great Ouse.

Stetson Pit, Dare County, North Carolina: An integrated chronologic, faunal, and floral record of subsurface coastal quaternary sediments

Continuous split spoon samples from a drill hole penetrating 34 m of coastal plain sediments at Stetson Pit in Dare County, North Carolina were taken for lithologic, aminostratigraphic, faunal (ostracodes) and floral (pollen) analyses. Three distinct aminozones are recognized in the subsurface section based upon D-alloisoleucine/L-isoleucine (A/I) values in each of the molluscan species Mulinia lateralis and Mercenaria sp. Ostracode zonations in the subsurface section are based on percentages of 80 thermophilic and cryophilic species (those living today south and north of Cape Hatteras) and the percentages of brackish water species. Five assemblage zones are delineated. Six pollen assemblage zones are also delineated within the subsurface section based upon study of 48 sediment samples. The subsurface record at Stetson Pit is interpreted to represent portions of four interglacials based upon the combined faunal, floral and aminostratigraphic data. The two younger aminozones, with amino acid age estimates of 100,000±20,000 yr (−7.2 to −11.2 m MSL) and 300,000–500,000 yr (−13 to −14.2 m MSL), represent portions of middle/late Pleistocene interglacials. The lower aminozone (−17.4 to −33 m MSL) spans an interval that probably includes at least two interglacials (based upon faunal and floral records) and has an age estimated to be between 800,000 and 1,300,000 yr. Boundaries delineated by faunal, floral, and amino acid methods do not always coincide, due to sampling constraints and phase lags between the different records. One major unconformity (at −17.4 m MSL) in the Stetson Pit section is easily recognized from lithologic characteristics and may represent a hiatus of as much as 800,000 yr. Lithologic changes associated with all other zone boundaries are subtle and would probably not be considered significant in the absence of faunal, floral, or amino acid data.

The precision of amino acid geochronology and paleothermometry

Uncertainties in the measurements necessary for the calculation of amino acid age estimates and paleotemperatures can be propagated through age and temperature equations to yield estimates of precision. It is found that aIle/Ile ratios can be used to calculate numerical ages with a precision generally no better than ±40–50% if the age equation is not calibrated locally and with a precision approaching 15% if appropriate calibration samples are available locally. The effective diagenetic temperature of a sample of known age can be calculated to within about 2°C, although the local relationship between ground and air temperatures may add additional uncertainty when interpreting the effective temperature in terms of climate. The effective temperature for an interval of time bracketed by samples of known age can be known to within about 2–4°C under favorable circumstances. The change in effective temperature between an interval of time bracketed by two samples of known age and the period of time following that interval can be calculated to within 1–2°C in may instances.

Amino acid age estimates of marine terraces and sea levels on San Nicolas Island, California

Fourteen marine terraces rising to an elevation of 274 m have been mapped on San Nicolas Island, California. The lowest terrace is ∼105 000 yr old, on the basis of amino acid ratios in fossil mollusks slightly lower than those from the ∼120 000 yr old, uranium-series dated second terrace. The age estimate and present elevation of the lowest terrace, along with an assumption of constant uplift rate, indicate a paleo–sea-level estimate of −10 to −12 m at about 105 000 B.P., supporting the general Barbados model of sea-level history. The fourth and fifth terraces probably represent separate high sea-level stands during the same interglacial complex ≥400 000 B.P. The eight and tenth terraces have amino acid ratios near equilibrium and imply ages of ≥600 000 yr. Collectively, the data suggest long-term average Quaternary uplift rates of 0.2–0.3 m/1000 yr.

Quaternary sea-level events on northern San Clemente Island, California

Global sea-level changes are expressed in the coastal landforms and deposits of northern San Clemente Island. Stratigraphic, radiometric, amino acid, and pedologic dating techniques have allowed the development of a chronology of sea-level changes for about the last 500,000 yr. A uranium-series date on coral of about 127,000 yr for the 2nd terrace serves as a calibration point for amino acid age estimates of four other mapped terraces. Two of these terraces have age estimates of about 80,000–105,000 yr, another has an age estimate of about 127,000 yr, and the 5th terrace on the west side of the island is estimated to be about 415,000–575,000 yr old. These dates correlate reasonably well with marine terraces dated elsewhere and with stages of the oxygen-isotope record that are thought to represent high stands of the sea. Weakly cemented calcareous dune sands (eolianites) are moderately extensive on northern San Clemente Island and appear to represent low stands of the sea, since calcareous shelf sands were the most likely source. A radiocarbon date of about 22,000 yr suggests that the youngest eolianite was deposited during the last glacial maximum. An older eolianite is estimated to be about 140,000–195,000 yr old based on stratigraphic relations and degree of soil development. The suggested ages for the eolianites also correlate well with oxygen-isotope estimates of low sea levels.

Amino acid age estimates, quaternary Atlantic Coastal Plain: Comparison with U-series dates, biostratigraphy, and paleomagnetic control

Amino acid enantiomeric ( D L ) ratios in the mollusk Mercenaria are compared with recently published biostratigraphic and/or U-series solitary coral data from 22 Quaternary localities on the central and southern Atlantic Coastal Plain. In all cases, local relative aminostratigraphic sequences are consistent with relative ages inferred from U-series or biostratigraphic data, although occasionally more depositional events are recognized by aminostratigraphic than biostratigraphic methods. However, if the U-series data are used as age calibrations for the D L values, latitudinal trends of “isochronous” D L values are highly variable and conflict with trends expected from the present temperature gradient, which is smooth and nearly linear between 45° and 25° N. Age estimation can be performed independently of the U-series data using a kinetic model that relies on the assumption that Pleistocene temperature gradients have also been smooth functions of latitude, although significantly steeper than the present temperature gradient. Within the uncertainties of this assumption, kinetic model age estimates for localities in the coastal plain fall into the following groups: 70,000–130,000 yr, 200–250,000 yr, 300,000–400,000 yr, 500,000–600,000 yr, 700,000–800,000 yr, and > 1,000,000 yr. Major conflicts between these model age estimates are observed for localities near Charleston, South Carolina and in central Virginia. These conflicts could indicate that the basic temperature assumptions of aminostratigraphy are incorrect, and that apparent local aminostratigraphic sequences (clusters of different D L values) could be due to factors other than age difference. Alternatively, some of the U-series dates may be only minimum ages for these localities.

Multiple glaciations and marine transgressions, western Kennedy Channel, Northwest Territories, Canada

Along a 70 km section of western Kennedy Channel three prominent weathering zones are identified and serve to differentiate major events in the Quaternary landscape. The oldest zone (Zone 111b) is characterized by a deeply weathered, erratic‐free terrain which extends from the mountain summits down to ca. 470 m above sea level. This zone shows no evidence of former glacierization. Zone 111a extends from ca. 470 to 370m above sea level and is characterized by sparse granite, gneiss and quartzite erratics amongst weathered bedrock and extensive, oxidized colluvium. The Precambrian provenance and uppermost profile of these erratics reflect the maximum advance of the northwest Greenland Ice Sheet onto northeastern Ellesmere Island. These uppermost erratics along western Kennedy Channel decrease in elevation southward and suggest that the former Greenland ice was thickest in the direction of the major outlet of Petermann Fiord. No evidence of a former ice ridge in Nares Strait was observed. Zone II is marked by the moraines of the outermost Ellesmere Island ice advance which form a prominent morpho‐stratigraphic boundary where they cross‐cut the zone of Greenland erratics at ca. 250–350 m above sea level. These moraines show advanced surface weathering and ice recession from them is associated with a pre‐Holocene shoreline at 162 m above sea level. Late Wisconsin/Würm glacial deposits, equivalent to Zone I, were not observed in the lower valleys bordering Kennedy Channel. The outermost Ellesmere Island ice advance (Zone II) is radiometrically bracketed by 14C dates on in situ shells from subtill and supratill marine units which are 40,350±750 and>39,000 B.P., respectively. Amino acid age estimates on the same shell samples and others from similar stratigraphic positions all suggest ages of >35,000 B.P. Stratigraphically and chronologically this ice advance is correlated with the outermost Ellesmere Island ice advance 20–40 km to the north which formed small ice shelves when the relative sea level was ca. 175 m above sea level. The Holocene marine transgression along western Kennedy Channel occurred in an ice‐free corridor maintained between the separated margins of the northwest Greenland and northeast Ellesmere Island ice sheets during the last glaciation. Initial emergence may have begun ca. 12,300 B.P., however, sea level had dropped only 15 m by ca. 8000 B.P. after which glacio‐isostatic unloading of the corridor was rapid. The implications of these data are discussed in the context of existing models on high latitude glaciation and paleoclimatic change

Glacial and Sea-Level History of Southwestern Cumberland Peninsula, Baffin Island, N.W.T., Canada

Southwestern Cumberland Peninsula has been glacierized at various times by eastward flowing Laurentide ice, by an expanded Penny Ice Cap, and by fiord and valley glaciers fed from cirques. Two major weathering zones (A and B) and a fivefold subdivision of the younger zone (A) are recognized. The zone boundaries are marked by end and lateral moraines, some of which are related to raised marine deltas. Radiocarbon dates and amino acid age estimates on fossils from these deltas and correlations with the stratigraphic record of northern Cumberland Peninsula allow establishment of a tentative absolute chronology. The oldest terrain is that glaciated before the last interglaciation. The last glaciation is subdivided locally into the Duval, Outer Usualuk and Ranger stades, named for the end and lateral moraines which mark their maximum extents. During the Duval Stade (ca. 100,000 to 60,000 yr BP), Laurentide ice filled the inner one-third of Cumberland Sound and ice from local sources filled Pangnirtung and Kingnait fiords. Relative sea level was 100 m above present at the ice mar

Anomalously high uplift rates along the Ventura—Santa Barbara Coast, California—tectonic implications

The NW—SE trending segments of the California coastline from Point Arena to Point Conception (500 km) and from Los Angeles to San Diego (200 km) generally parallel major right-lateral strike-slip fault systems. Minor vertical crustal movements associated with the dominant horizontal displacements along these fault systems are recorded in local sedimentary basins and slightly deformed marine terraces. Typical maximum uplift rates during Late Quaternary time are about 0.3 m/ka, based on U-series ages of corals and amino-acid age estimates of fossil mollusks from the lowest emergent terraces. In contrast, the E–W-trending segments of the California coastline between Point Conception and Los Angeles (200 km) parallel predominantly northward-dipping thrust and high-angle reverse faults of the western Transverse Ranges. Along this coast, marine terraces display significantly greater vertical deformation. Amino-acid age estimates of mollusks from elevated marine terraces along the Ventura—Santa Barbara coast imply anomalously high uplift rates of between 1 and 6 m/ka over the past 40 to 100 ka. The deduced rate of terrace uplift decreases from Ventura to Los Angeles, conforming with a similar trend observed by others in contemporary geodetic data. The more rapid rates of terrace uplift in the western Transverse Ranges reflect N—S crustal shortening that is probably a local accommodation of the dominant right-lateral shear strain along coastal California.

Ice-age plant refugia in east Greenland

From the distribution of plants it has been inferred by some botanists that ice-free areas existed in East Greenland accomodating a flora which survived one or several ice ages in the area. Comparing this evidence with recent information on the chronology of glaciations and post-glacial vegetation development, there is both significant agreement and disagreement. The early hypothesis of survival of organisms since Tertiary times is refuted by the ubiquitous occurrence of glacigene deposits. However, some of the areas pointed out as sites for survival have remained ice-free longer than adjacent parts of Greenland. 14C dating and amino-acid age estimates of marine sediments show that lowland areas near the outer coast have been ice-free for at least 40,000 years. The vegetation history, as reflected in pollen diagrams extending back to ca. 10,000 yr. B.P., has shown that many of the extant species immigrated from northern Europe and North America in post-glacial times. This contingency includes both some thermophilous species that were suggested as survivors by one group of botanists, and some extremely “hardy” species that were thought to have survived by another group. From the palynological evidence it is inferred that the flora in the refugia comprised mainly species which today occur over a wide geographical and ecological range. The “odd” occurrences that initiated the discussion may represent random seed dispersal accumulated in the ice-free areas through long periods of time.

Former Ice Shelves in the Canadian High Arctic

AbstractMoraines deposited by the outermost ice advance across Judge Daly Promontory, northeastern Ellesmere Island, reflect thin, topographically controlled ice lobes extending to sea-level. The termini of two ice lobes were investigated and both produced ice shelves where they flowed into isostatically depressed embayments along western Kennedy Channel. Morphological evidence for these ice shelves occurs at the entrance to these valleys where steeply descending lateral moraines become abruptly horizontal for 2 km. In addition, both the horizontal moraines and associated pro-glacial terraces are fossiliferous down-valley from the apparent grounding line. Based on the differences in elevation between the horizontal moraines and the valley bottoms, the two ice shelves had estimated thicknesses ofc. 110 and 150 m. A proglacial outwash terrace at 175 m a.s.l. is considered to represent the approximate relative sea-level during the formation and break-up of the ice shelves. This relative sea-level is consistent with the water depths required to float the calculated ice thicknesses in both valleys. Associated with these ice margins are finite14C dates of 28 000-30 000 B.P. and amino-acid age estimates of >35 000 B.P. The importance and likelihood of additional past ice shelves in the Canadian High Arctic is discussed.

Former Ice Shelves in the Canadian High Arctic

AbstractMoraines deposited by the outermost ice advance across Judge Daly Promontory, northeastern Ellesmere Island, reflect thin, topographically controlled ice lobes extending to sea-level. The termini of two ice lobes were investigated and both produced ice shelves where they flowed into isostatically depressed embayments along western Kennedy Channel. Morphological evidence for these ice shelves occurs at the entrance to these valleys where steeply descending lateral moraines become abruptly horizontal for 2 km. In addition, both the horizontal moraines and associated pro-glacial terraces are fossiliferous down-valley from the apparent grounding line. Based on the differences in elevation between the horizontal moraines and the valley bottoms, the two ice shelves had estimated thicknesses ofc. 110 and 150 m. A proglacial outwash terrace at 175 m a.s.l. is considered to represent the approximate relative sea-level during the formation and break-up of the ice shelves. This relative sea-level is consistent with the water depths required to float the calculated ice thicknesses in both valleys. Associated with these ice margins are finite14C dates of 28 000-30 000 B.P. and amino-acid age estimates of >35 000 B.P. The importance and likelihood of additional past ice shelves in the Canadian High Arctic is discussed.