A younger glacial Lake Iroquois in the Lake Ontario basin, Ontario and New York: re-examination of pollen stratigraphy and radiocarbon dating

Revision of palynochronologic and radiocarbon age estimates for the termination of glacial Lake Iroquois, mainly based on a currently accepted younger determination of the key Picea–Pinus pollen transition, shows agreement with recently established constraints for this late glacial event in the Lake Ontario basin at 13 000 cal years BP. The date of emergence or isolation of small lake basins reflects the termination of inundation by glacial lake waters. The increasing upward presence of plant detritus and the onset of organic sedimentation marks the isolation level in the sediments of a small lake basin. The upward relative decline and cessation of pollen from trees such as Pinus, Quercus, and other thermophilous hardwoods that were wind transported long distances from southern areas also mark the isolation of inundated small lake basins by the declining water level of Lake Iroquois as local vegetation grew and local pollen overwhelmed long-distance-transported pollen. Re-examination of data in small lake basins north of Lake Ontario using the above criteria shows that the age range for the termination of Lake Iroquois derived from these data overlaps other age constraints. These constraints are based on a varve-estimated duration of post-Iroquois phases before incursion of the Champlain Sea, a newly discovered late ice advance into northern New York State, and the age of a mastodon at Cohoes, New York. The new age (13 000 cal years BP) for Lake Iroquois termination is significantly younger than the previous estimate of 11 800 14C (13 600 cal) years BP.

A high-resolution ostracod-derived δ18O record of early Holocene abrupt climatic change from N. Scotland.

<p>Oxygen-isotope ratios can be measured on a range of materials (e.g. ostracods, bulk carbonates, diatom silica) and this, alongside their sensitivity to changes in temperature and precipitation has resulted in oxygen-isotope analyses becoming a well-established tool for investigating palaeoclimatic change. We use δ<sup>18</sup>O of calcite from ostracod shells to reconstruct palaeotemperature and palaeo-precipitation variability during an early Holocene abrupt climatic event in Crudale Meadow, SW Orkney Mainland, Scotland, UK. The research ultimately aims to further our understanding of the driving mechanisms of palaeoclimatic change during the early Holocene by producing a high-resolution palaeoclimate record from Crudale Meadow and comparing this to the existing data of NW Europe. </p><p>Crudale Meadow is an ideal study site for this research. Spatially, it completes a transect of published early Holocene δ<sup>18</sup>O records that span Western Ireland<sup>1</sup>, NW England<sup>2</sup> and into Scandinavia<sup>3</sup>. It has a ~3m thick early Holocene carbonate sequence which offers a multi-decadal or multi-centennial scale study resolution and its proximity to the N. Atlantic makes it highly likely to have been influenced by any climatic changes in the region. A previous study<sup>4</sup> presented a bulk carbonate δ<sup>18</sup>O record for Crudale Meadow but the skeletal chronology limits its usefulness for comparing with regional trends. Here, we present an improved chronology using  tephra and pollen stratigraphy, in addition to the ostracod-derived δ<sup>18</sup>O record. The studied sequence is anchored by the previously identified Saksunarvatn visible tephra layer dated to 10,210 ± 70 cal. years BP<sup>5</sup>.</p><p>Ostracods are micro-crustaceans with low-Mg calcite shells which take on the isotopic signal of the water body they are in, at the time of shell calcification. In this study, we use winter calcifying <em>Candona</em> spp. for isotopic analysis. These were abundant and well preserved throughout the sequence. Members of this genus have a well-characterised vital offset<sup>6</sup> so the δ<sup>18</sup>O curve can be reliably corrected for vital effects. Moreover, the species analysed are probable winter calcifers, thus reducing the impact of isotopic enrichment through lake water evaporation during summer months. The high-resolution study allows us to identify structure within the identified isotopic excursion and suggest palaeotemperature estimates from the ostracod- and chironomid-inferred temperatures.</p><p>The new data presents a clear climatic event with internal structure, which with the current chronology, we hypothesise to be the 9.3ka event. The 9.3ka event has fewer detailed records in comparison to other early Holocene abrupt climatic events (e.g. 8.2ka). Consequently, to identify a structured isotopic signal of the 9.3ka event in NW Europe is an important contribution to our early Holocene records. It emphasises the need for high-resolution δ<sup>18</sup>O studies during the early Holocene across NW Europe in order to be able to fully identify subtle abrupt climatic events. </p><p><strong>References</strong>: <sup>1</sup>Holmes, J.H. <em>et al.</em> (2016) <em>QSR</em>, p.341-349; <sup>2</sup>Marshall, J.D. <em>et al. </em>(2007) <em>Geology</em>, 35, p.639–642; <sup>3</sup>Hammarlund, D. <em>et al.</em> (2002) <em>The Holocene,</em> 12, p.339–351; <sup>4</sup>Whittington, G. <em>et al.</em> (2015) <em>QSR</em>, 122, p.112–130;<sup> 5</sup>Timms, R.G.O. <em>et al</em>. (2018) <em>Quat. Geochron.</em> 46, p.28–44; <sup>6</sup>Holmes, J.H & Chivas, A. (2002) <em>AGU Geophysical Monograph,</em> p.118-204.</p>

Small shards and long distances – three cryptotephra layers from the Nahe palaeolake including the first discovery of Laacher See Tephra in Schleswig-Holstein (Germany)

Investigations of Lateglacial to Early Holocene lake sediments from the Nahe palaeolake (northern Germany) provided a high-resolution palynological record. To increase the temporal resolution of the record a targeted search for cryptotephra was carried out on the basis of pollen stratigraphy. Three cryptotephra horizons were detected and geochemically identified as Saksunarvatn Ash, Vedde Ash and Laacher See Tephra. Here we present the first geochemically confirmed finding of the ash from the Laacher See Eruption in Schleswig-Holstein – extending the so far detected fallout fan of the Lateglacial eruption further to the North-West. These finds enable direct stratigraphical correlations and underline the potential of the site for further investigations.

Vegetation history along the eastern, desert escarpment of the Sierra San Pedro Mártir, Baja California, Mexico

Plant macrofossils from 38 packrat middens spanning the last ~ 33,000 cal yr BP record vegetation between ~ 650 and 900 m elevation along the eastern escarpment of the Sierra San Pedro Mártir, northern Baja California. The middens span most of the Holocene, with a gap between ~ 4600 and 1800 cal yr BP, but coverage in the Pleistocene is uneven with a larger hiatus between 23,100 and 14,400 cal yr BP. The midden flora is relatively stable from the Pleistocene to Holocene. Exceptions include Pinus californiarum, Juniperus californica and other chaparral elements that were most abundant > 23,100 cal yr BP and declined after 14,400 cal yr BP. Despite being near the chaparral/woodland-desertscrub ecotone during glacial times, the midden assemblages reflect none of the climatic reversals evident in the glacial or marine record, and this is corroborated by a nearby semi-continuous pollen stratigraphy from lake sediments. Regular appearance of C4 grasses and summer-flowering annuals since 13,600 cal yr BP indicates occurrence of summer rainfall equivalent to modern (JAS average of ~ 80–90 mm). This casts doubt on the claim, based on temperature proxies from marine sediments in the Guaymas Basin, that monsoonal development in the northern Gulf and Arizona was delayed until after 6200 cal yr BP.

Pollen analysis: state of the art

Although nearly 50 years have passed since P.B. Sears introduced pollen analysis to North America, it remains an occult art. Dramatic improvements in sampling and analytic techniques continue to be limited by intractable problems of differential production, dispersal, ballistics, sedimentation, and preservation. It is a basic tenet of pollen stratigraphy that the data set, consisting primarily of microfossils preserved in sediments, is better than anything we have yet been able to do with it. Basic agreement between late- and postglacial pollen records has been confirmed wherever the method has been applied. Quantitative sampling techniques, sample preparation, and analytic procedures, together with multiple radiocarbon dates, permits calculation of sedimentation rates and absolute pollen influx. Of approximately 300 sediment cores from northeastern North America, fewer than 30 have more than 3 radiocarbon determinations from which least squares power curve regressions can be reliably calculated in the determination of sedimentation rates. Analogy with modern environments represented by surface pollen spectra is limited by an insufficient number of samples of uniform quality to characterize a vegetational mosaic covering 40 degrees of latitude (40-80°N) and longitude (60-100°W). The present surface pollen data bank includes about 700 samples, unevenly spaced and of uneven quality, permitting a grid resolution of no better than 10,000 km2.