Intertidal Peat Research Articles

Mid- and late-Holocene environmental change in western Ireland: New evidence from coastal peats and fossil timbers with particular reference to relative sea-level change

The results of radiocarbon dating and dendrochronological investigations of timbers (mainly pine and oak) preserved in coastal peats on the Atlantic coast of mid-western Ireland are reported. These are complemented by the results of pollen analytical investigations, including a detailed pollen profile from Lough Atalia, a brackish lake located within present-day Galway city and immediately to the east of the medieval town. The results from the fossil timber investigations show that pine stumps exposed on coastal intertidal peats in mid-western Ireland date mainly to ca. 3000 BC, that is, they are coeval with the so-called ‘pine flushes’ on blanket bog in western Ireland. Many of the oak timbers are considerably older while an oak on peat in Turlin Bay, near Silverstrand, persisted until ca. 2100 BC. This fossil timber record, taken in conjunction with the pollen records, suggests conditions very different to those pertaining at present, including absence of direct marine influence because of relative sea levels (RSLs) that were probably at least 2 m, and possibly up to 4 m, lower than at present. Mid-Holocene conditions were less stormy, and lower precipitation and/or higher evapotranspiration facilitated tree colonisation of peat surfaces. Intertidal peat layers at Salthill show that present-day RSL began to be approached at ca. AD 500. This agrees with the evidence from An Loch Mór, Inis Oírr, which indicates that salinity in this brackish lake began to increase noticeably at/immediately prior to ca. AD 1000. A notable feature of the pollen record from L. Atalia is a yew ( Taxus)-dominated woodland phase of ca. 600 years duration centred on 2550 BC. A major and widespread expansion of yew at about this time is a distinctive feature of mid-Holocene woodlands in western Ireland.

Revealed by Waves: A Stratigraphic, Palaeoecological, and Dendrochronological Investigation of a Prehistoric Oak Timber and Intertidal Peats, Bay of Ireland, West Mainland, Orkney

ABSTRACTAn oak timber was discovered in 2013 within intertidal peats at the Bay of Ireland, Stenness, Orkney, representing a unique archaeological find. Subsequent excavation and rescue of the timber took place in 2014 to investigate its stratigraphical relationship before further eroding. Dendrochronological and morphological study identified the timber as a possible radially split log, c. 150 years of age when felled. No dendrochronological match was possible, and a wiggle-match date obtained provided a Later Mesolithic felling date of 4410–4325 cal BC. This timber is the first and only evidence so far for the use of oak in Mesolithic Orkney. The timber is significant palaeoecologically, suggesting oak may have been indigenous to Orkney. This adds to a growing argument for the existence of areas of “true woodland”. Pollen evidence shows the timber was deposited within reedswamp, fringed by willow-birch carr-woodland, with oak unlikely to have been growing in the immediate location. High microscopic and macroscopic charcoal values indicate Later Mesolithic communities exerted influence on this wetland using burning as a tool for landscape modification. It is unknown what the timber represents; it may have been for construction purposes or as a marker/possible landing place showing the path to the Loch of Stenness.

A Late Post-Glacial environmental record and Early Neolithic intertidal peat land surface at Porth Neigwl, Llŷn Peninsula, Gwynedd

A Late Post-Glacial environmental record and Early Neolithic intertidal peat land surface at Porth Neigwl, Llŷn Peninsula, Gwynedd

Spring migration of red knots along the Virginia barrier islands

ABSTRACTWe conducted low‐altitude, aerial surveys from the last week of April through the first week of June (1995, 1996, 2005–2013) to assess patterns in bird abundance, migration phenology, and habitat use of red knots (Calidris canutus rufa) staging along the Virginia barrier islands. During most years, knots arrived in early May, reached peak abundance during the third week of May, and were gone by mid‐June. The peak period (10 May–3 June) accounted for >90% of detections for all years. The mean high count during the study was 7,373 ± 407 (SE). Decadal comparisons of high counts, accumulated numbers for the period of peak abundance and the season totals provided mixed results. Knots within the study area used peat banks and sandy beaches for foraging. Intertidal peat banks were patchily distributed and represented only 6% of the shoreline. Mean knot densities were comparable between the 2 habitats in the early period of migration but diverged rapidly such that by the third week of May, knot densities were 10‐fold higher on peat compared to sand. Knots exhibited wide annual variation in habitat use; the number of knots observed on peat varied from 13% to 91% of the total birds for a given year, presumably reflecting the underlying population dynamics of respective bivalve populations. Potential prey dynamics coupled with the patchy distribution of peat appear to cause year‐to‐year variation in space use along the island chain. The Virginia barrier islands appear to be part of a terminal staging area for red knots that stretches from New Jersey south through the Outer Banks of North Carolina. Documenting red knot use within portions of this region beyond the well‐studied Delaware Bay may lead to a more complete understanding of the sampling process used by red knots to make staging decisions. © 2014 The Wildlife Society.

Age and palaeoenvironmental significance of an inter-tidal peat bed at Ballywoolen, Bann estuary, Co. Londonderry

A recently exposed inter-tidal peat bed at Ballywoolen, Bann estuary, Co. Londonderry, has yielded new information about mid-Holocene coastal environmental change in the north-east of Ireland. Pollen analytical data and wood detritus demonstrate that peat accumulation occurred in a terrestrial environment that was free from marine influence. Radiocarbon dates suggest that the peat accumulated rapidly during a period of low relative sea level subsequent to the maximum of Holocene relative sea-level rise along the north coast of Northern Ireland. The absence of marine/brackish indicator taxa at the site suggests that the tidal range was somewhat less than that at present and/or that the channel of the river was located some distance east of its present alignment. The dates indicate that the low stand lasted for at least ~0.2 ka and possibly for ~1.1 ka. Stable, woodland-dominated landscapes are indicated at both this site and neighbouring ones around ~6.4-5.3 cal ka BP. There is no evidence for large-scale aeolian sand movement or human impact on the landscape during the period of peat accumulation.

Radiocarbon dates from the inter-tidal peat bed at Portrush, County Antrim

Two new 14C dates from a bed of compact inter-tidal peat at Mill Strand, Portrush, are reported and discussed in relation to two previously obtained ages from peat at Dhu Varren, a short distance inland. The date from the top of the Mill Strand peat is statistically inseparable from that of the peat top at Dhu Varren. Together these dates indicate the cessation of peat growth occurred _6950-6550 cal. BP as a result of sand deposition associated with sea-level rise. However, dated samples from deeper within the respective stratigraphies are not compatible. Their respective ages in relation to their elevations pose a problem of interpretation.

Radiocarbon dates from the inter-tidal peat bed at Portrush, County Antrim

Two new 14C dates from a bed of compact inter-tidal peat at Mill Strand, Portrush, are reported and discussed in relation to two previously obtained ages from peat at Dhu Varren, a short distance inland. The date from the top of the Mill Strand peat is statistically inseparable from that of the peat top at Dhu Varren. Together these dates indicate the cessation of peat growth occurred ~6950-6550 cal. BP as a result of sand deposition associated with sea-level rise. However, dated samples from deeper within the respective stratigraphies are not compatible. Their respective ages in relation to their elevations pose a problem of interpretation.

Age and palaeoenvironmental significance of an inter-tidal peat bed at Ballywoolen, Bann estuary, Co. Londonderry

Abstract A recently exposed inter-tidal peat bed at Ballywoolen, Bann estuary, Co. Londonderry, has yielded new information about mid-Holocene coastal environmental change in the north-east of Ireland. Pollen analytical data and wood detritus demonstrate that peat accumulation occurred in a terrestrial environment that was free from marine influence. Radiocarbon dates suggest that the peat accumulated rapidly during a period of low relative sea level subsequent to the maximum of Holocene relative sea-level rise along the north coast of Northern Ireland. The absence of marine/brackish indicator taxa at the site suggests that the tidal range was somewhat less than that at present and/or that the channel of the river was located some distance east of its present alignment. The dates indicate that the low stand lasted for at least~0.2 ka and possibly for~1.1 ka. Stable, woodland-dominated landscapes are indicated at both this site and neighbouring ones around~6.4–5.3 cal ka BP. There is no evidence for large-...

Probable Neolithic footprints preserved in inter-tidal peat at Kenfig, South Wales (UK)

Holocene footprints have been reported from several locations around the UK coast preserved within inter-tidal sediments. These sediments are normally fine-grained silts, sands and clays. Here we report potential human footprints preserved in the lower of two inter-tidal peat units exposed on the foreshore at Kenfig in South Wales. The lower peat layer pre-dates 3810±40 BP and its maximum age is constrained by two dates, one of 4930±50 BP and another of 5110±50 BP. The prints may have formed at any point within this time interval. Both peat units formed in a salt marsh, or fen environment, as indicated by the analysis of both diatoms and pollen and have been tentatively assigned to the middle Wentlooge Formation within the Holocene lithostratigraphy established for the Severn Estuary Holocene sequence. The footprints were digitised using an optical laser scanner and are compared to other Holocene human footprints exposed on the Sefton Coast in North West England. In comparison the Kenfig prints have poor anatomical form which may be the result of both the substrate and post-depositional modification. The paper demonstrates the value of using optical laser scanning in the data capture and analysis of transitory archaeological and geological trace evidence.

Offshore and intertidal peat deposits, England — a resource assessment and development of a database

In response to an enhanced interest in submerged landscapes and their nature, use and exploitation by early inhabitants, a multidisciplinary inventory of intertidal, nearshore and offshore peat deposits and ‘submerged forests’ around the English coast was compiled. The main driver was to assess the research potential of these types of deposits, in terms of their number, distribution and characteristics in order to identify recording priorities, especially where there is risk of either loss by erosion or of further submergence. The data collected included detailed site descriptions, locations, altitudes relative to modern sea-level, associated archaeological artefacts and any radiocarbon ages, as well as bibliographic references. Information was gathered from published sources, grey literature and the National Monuments Record, although some data (particularly those associated with offshore developments and aggregate extraction) remain confidential. At present the database contains information on over 300 sites, stored electronically. It constitutes a centralised resource to facilitate future coastal and marine research projects. Currently the database is only accessible on a one-to-one basis, but there are plans to make it available to users online. The project successfully involved the amalgamation of information from diverse sources and resulted in bringing together the often-segregated disciplines of Archaeology, Geology and Geography, with the hope of strengthening these relationships in the future.

Reply to Gischler E, Comment on Toscano and Macintyre (2005): corrected western Atlantic sea-level curve for the last 11,000 years based on calibrated 14C dates from Acropora palmata framework and intertidal mangrove peat Coral Reefs 22: 257–270 (2003), and their response in Coral Reefs 24:187–190 (2005)

Reply to Gischler E, Comment on Toscano and Macintyre (2005): corrected western Atlantic sea-level curve for the last 11,000 years based on calibrated 14C dates from Acropora palmata framework and intertidal mangrove peat Coral Reefs 22: 257–270 (2003), and their response in Coral Reefs 24:187–190 (2005)

Intertidal peats and the archaeology of coastal change in the Severn Estuary, Bristol Channel and Pembrokeshire

Abstract Dates for the beginning and end of intertidal and coastal peat formation are reviewed in the Severn Estuary, Bristol Channel and Pembrokeshire. Peat formation at many sites started between c. 6000–4000 Cal bc and in the Severn Estuary continued until c. 200 Cal bc . Archaeological evidence is concentrated at two main stages within the coastal sequences. Throughout the area Mesolithic sites underlie the earliest peat and relate to coastal exploitation just prior to the transgression represented by peat formation. During the main period of peat formation from the later Mesolithic to the early Bronze Age there is only small-scale human activity within the coastal peats. A second episode of concentrated human activity is confined to the Severn Estuary and occurs in the middle Bronze Age and Iron Age. Round and rectangular buildings and trackways are associated with the initial stages of marine transgressions which led to the burial of a coastal bog by minerogenic silts. A transgression in the middle Bronze Age c. 1400 Cal bc was followed by a regression phase, the main period of human activity at Goldcliff, this ended with a widespread transgression centred on in the third century bc . The factors which attracted human activity at particular stages within the coastal sequence are considered, as is the relative visibility of human activity during each sedimentary stage. The contribution which archaeological evidence, particularly dendrochronological dating of wooden structures, can make to the dating of coastal change is emphasized.

A 5000year record of intertidal peat stratigraphy and sea level change from northwest Alaska

Abstract Coastal environments of northwest Alaska preserve a detailed record of sea level change during the past 5000 14 C yr. Rapid burial of intertidal marshes by storm overwash processes, a short open water period, and the arctic maritime climate contribute to the preservation of marine and eolian facies. Eustatic and storm-controlled changes in sea level can be identified from interbedded sequences of marsh peat and coastal flood deposits on barrier islands and estuaries along northwest Seward Peninsula. Mean eustatic sea level has risen about 1.5 m during the last 5000 years, at an average of ca. 0.028 cm yr−1, based on the depth and age relationship of a suite of 23 peat horizons sampled from a 140 km-long reach of coast. Nearshore erosion and sedimentation are controlled by secular variations in wave climate. Peaks in tidal amplitude and storminess correlate with transgressive sedimentary regimes and occurred during the periods 3300–1700, 1200–900, and since about 400 14 C yr BP. Short-term fluctuations in relative sea level are superimposed on the long-term regional eustatic trend, and record the coastal response to variable rates of sea-level change during the late Holocene.

Seismic or hydrodynamic control of rapid late-Holocene sea-level rises in southern coastal Oregon, USA?

Intertidal stratigraphy has been instrumental in demonstrating the hazard posed by great earthquakes at the Cascadia subduction zone, but inferring an earthquake history from interbedded sequences of peat and mud is complicated by many factors that influence sedimentation and relative sea-level change on both tectonic and nontectonic coasts. Rapid-to-sudden rises in relative sea level marked by sharp contacts between intertidal peat and overlying mud or sand may reflect coseismic coastal subsidence and tsunami deposition or, alternatively, nonseismic hydrodynamic changes in estuaries. Reconnaissance coring at 16 sites in the marshes fringing a narrow, protected tidal inlet of Coos Bay, supplemented by diatom and 14C analyses at four sites, reveals a stratigraphic record too fragmentary and ambiguous to distinguish seismic from hydrodynamic causes for more than three of the 10 rises in relative sea-level identified. Only three sharp contacts have the wide extent and evidence of substantial (>0.5 m) submergence that distinguish them from similar contacts produced by nonseismic processes. Correlation with stratigraphic sequences at other estuaries shows that the fringing marshes suddenly subsided and were partially buried by tsunami sand during a great plate-boundary earthquake about 300 years ago. Similar contacts were produced by earthquakes about 1500–1800 years ago, and perhaps about 2400–2700 years ago. Other earthquakes with substantially less subsidence may also have occurred, but evidence is too ambiguous to reconstruct a more complete history.

Prehistoric intertidal archaeology and environments in the Severn Estuary, Wales

Estuaries, the interface between riverine systems and coastal environments, have great archaeological potential for interdisciplinary research. Extensive survey of 25km of coast in the Severn Estuary, Wales, databased using Autocad and GIS, is combined with more detailed investigation of 3km at Goldcliff. This demonstrates that coastal wetlands can preserve a diverse range of site types which contrasts with both terrestrial dryland contexts and other wetlands, such as the nearby Somerset Levels. Rectangular Bronze and Iron Age buildings, unique in Britain, trackways and other post settings are exposed on intertidal peat shelves. Activity on these former wetlands was particularly concentrated at the interface with marine flooding in the later Bronze Age and Iron Age, periods of major environmental change in both estuarine and riverine environments of the Severn. Activity seems to have been essentially opportunistic and seasonal, with evidence for cattle grazing and perhaps fishing. Marine inundation in the Iron Age was followed by Romano‐British reclamation.

Evolution of a machair landscape: pollen and related studies from Benbecula, Outer Hebrides, Scotland

ABSTRACTLow altitude sandy plains (machair) are a common feature of the Atlantic coasts of the Outer Hebrides. They formed as a result of shoreward movement of material consequent upon a rise in Holocene sea levels. A number of earlier, partially inconclusive, investigations into machair evolution proposed that fuller understanding could arise from an examination of inter-tidal organic deposits, in so far as these could provide a fossil proxy record of the processes involved in machair formation. Study of a series of inter-tidal peats sites located on a beach platform at Borve, island of Benbecula, provided both spatial and chronological evidence for sand movement as well as furnishing new data on vegetational and environmental history. The pollen diagrams show a flora in whichCalluna vulgaris(heather) and Poaceae (grass) are frequent dominants, while such arbroeal taxa asBetula(birch) andCorylus avellana-type(cf. hazel) are notable. The existence of birch–hazel woodland for the periodc.8855–6190 BP might conceivably have had a retarding effect on sand movement. Later cycles of sand movement would not have met such obstruction with the consequence that sand mobility and machair plain evolution could have been faster. Sand arrival at the seaward site (Borve 3), is dated toc.6735 BP and it continued untilc.6045 BP, after which it slowed before increasing again from around 5990 BP. This sand incursion produced a machair plain over the Borve sites as part of a continuous, but variously paced, marine and aeolian process. The presence of charcoal from c. 6860 BP, with a major increase in charcoal to pollen ratios by 6190 BP, may suggest that natural or intentional burning of the vegetation cover of the machair occurred in Mesolithic times; if the burning was anthropogenic in origin, then it pushes the possible involvement of humans in machair destabilisation to a time long before the previously proposed Neolithic period. A wet slack deposit, dated to 3400 ± 70 BP, indicates a time when sand movement overwhelmed the area around the landward site (Borve 1) and when the rate of sand movement was likely to have been impeded. The data from Borve suggest that evolution of machair landforms occurs at a slow, variable rate and that there is considerable long-term stability.

Coastal peats from northwest Ireland: implications for late‐Holocene relative sea‐level change and shoreline evolution

Peat and organic rich sediments at coastal sites in extreme northwest Ireland have accumulated in a wide variety of environments, often strongly influenced by late Holocene changes in relative sea level and by geomorphic processes. A deep peat sequence on the coast of Aranmore Island accumulated initially in a lake and subsequently in a freshwater marsh environment. The long pollen record serves as a template for regional events. It extends over much of the Holocene and shows relatively high levels of Pinus pollen up to just before the disappearance of this taxon at c. 3600 BP. Coastal peat occurrences elsewhere are much thinner and have accumulated over shorter periods; they contain further evidence to show that coastal areas were well‐wooded compared with today, and that Pinus was an important woodland component prior to c. 4000 BP. At sites in Gweebarra Bay intertidal peats record the closure of small estuaries by geomorphological events during the past 5000 years. Coastal sites at Ballyness, Clonmass, and Trawenagh display regressive stratigraphies ˜ minerogenic marine sediments are overlain by silty peats capped by highly organic freshwater peats. Basal radiocarbon dates range from 4500 to 3300 BP. The silty peats are interpreted as having formed in salt‐marsh environments and contain distinctive pollen spectra, marked by high levels of Pinus and Compositae Liguliflorae pollen. The data suggest that relative sea level attained levels close to that of today by the mid‐Holocene in this region. The pattern of relative sea‐level change agrees well with that predicted by geophysical modelling.

Holocene Intertidal Deposits of Sacramento-San Joaquin Delta, California: ABSTRACT

Rivers draining nearly one third of California reach sea level in the Sacramento-San Joaquin delta. Though tapered toward a constricted outlet and separated from the sea by a chain of estuarine bays and straits, the delta resembles deltas built into open-marine environments in that (1) its principal landforms--tidal wetlands and natural levees--are typical features of such delta, and (2) the fundamental process of deltaic deposition--discharge into standing water--affected the delta during historic floods. Tidal wetlands (marshes and swamps), generally served by fresh water, covered most of the delta (about 1,400 sq km) before agricultural reclamation. Rising sea levels began to create such wetlands at the site of the delta around 7,000 years ago. Continued sea level rise caused the wetlands to spread across flood plains of the Sacramento and San Joaquin Rivers, alluvial fans of lessor tributaries, and fields of eolian dunes. Once established, these wetlands built upward, apparently by accumulation of roots and rhizomes in growth position, to keep pace with sea level. Inorganic sediment largely bypassed the delta and settled instead in the brackish-and salt-water bays to the west. Accordingly, the typical vertical sequence of facies in the delta consists of intertidal peat overlying alluvium or dune sand. Principal exceptions are the sands and muds of channels, the silts of natural levees along distributaries of the Sacramento River, and the clays of tidal wetlands near these distributaries. Major high stands of the sea during Pleistocene time should have produced intertidal peat at the site of the delta. The scarcity or absence of Pleistocene peat implies removal by streams, wind, exudation, and/or anaerobic decomposition. The latter three agents have removed Holocene peat from farmed, reclaimed wetlands of the delta as rapidly as 7 cm/year. End_of_Article - Last_Page 438------------

MID‐FLANDRIAN CHANGES IN VEGETATION ON MAINLAND ORKNEY

SummaryThe presence of birch‐hazel scrub with a rich ground flora of tall herbs and ferns in the mid‐Flandrian on Mainland, Orkney, suggested by Moar (1969), is supported, but no evidence was obtained for Pinus, Quercus, Alnus or other major forest trees as indigenous species. Evidence from lake muds and intertidal peat in the region of the Bay of Skaill suggests an increase in onshore wind speeds with the formation of the Bay of Skail and the beginning of sand‐blow c. 5000 B.P. The birch‐hazel scrub declined at this time and the remaining tall herb and fern vegetation was replaced by pasture within c. 200 years under the influence of neolithic man and his grazing animals. At c. 3400 B.P. a combination of increased oceanicity of climate and high grazing pressure appear to have led to the initiation of blanket peat formation on the hills. A contribution to the interpretation of 14C dates derived from marly sediments is included.

Seasonal and Environmental Variations in Sediment Accretion in a Long Island Salt Marsh

Flax Pond is a small (0.5 km2) salt marsh on the north shore of Long Island, New York. Two 1 m2 plots within each of the following environments were covered with a marker layer of either brick dust or aluminum glitter: 1) bare mud flats; 2) areas newly colonized by Spartina alterniflora; and 3) high intertidal. S. alterniflora peat surfaces. Monthly cores revealed the amount of sediment that accumulated since placement of the marker. Accretion rates from October, 1974 to February, 1976 were as follows: bare mud flats −20.5 to 45.5 mm/yr; recently vegetated mud flats −9.5 to 37.0 mm/yr; and high intertidal peat surfaces −2.0 to 4.25 mm/yr. Sedimentation rates decrease with increasing elevation because of the reduced tidal submergence time and decreased height of the overlying water column. In areas of low elevation, ice and storms cause either erosion or a reduced rate of accretion during the winter months. The average mud accretion rate over the past 173 years is 3.4 mm/yr. Differences between the short-term rate and the long-term rate indicate substantial annual variation in the accumulation of mud in salt marshes. Short-term rates of peat accretion are similar to long-term estimates, indicating that rates of peat accretion are relatively constant over long intervals.