Post-glacial sea-level change along the Pacific coast of North America

Sea-level history since the Last Glacial Maximum on the Pacific margin of North America is complex and heterogeneous owing to regional differences in crustal deformation (neotectonics), changes in global ocean volumes (eustasy) and the depression and rebound of the Earth\u27s crust in response to ice sheets on land (isostasy). At the Last Glacial Maximum, the Cordilleran Ice Sheet depressed the crust over which it formed and created a raised forebulge along peripheral areas offshore. This, combined with different tectonic settings along the coast, resulted in divergent relative sea-level responses during the Holocene. For example, sea level was up to 200 m higher than present in the lower Fraser Valley region of southwest British Columbia, due largely to isostatic depression. At the same time, sea level was 150 m lower than present in Haida Gwaii, on the northern coast of British Columbia, due to the combined effects of the forebulge raising the land and lower eustatic sea level. A forebulge also developed in parts of southeast Alaska resulting in post-glacial sea levels at least 122 m lower than present and possibly as low as 165 m. On the coasts of Washington and Oregon, as well as south-central Alaska, neotectonics and eustasy seem to have played larger roles than isostatic adjustments in controlling relative sea-level changes

A post-glacial sea level hinge on the central Pacific coast of Canada

Post-glacial sea level dynamics during the last 15,000 calendar years are highly variable along the Pacific coast of Canada. During the Last Glacial Maximum, the Earth\u27s crust was depressed by ice loading along the mainland inner coast and relative sea levels were as much as 200 m higher than today. In contrast, some outer coastal areas experienced a glacial forebulge (uplift) effect that caused relative sea levels to drop to as much as 150 m below present levels. Between these inner and outer coasts, we hypothesize that there would have been an area where sea level remained relatively stable, despite regional and global trends in sea level change. To address this hypothesis, we use pond basin coring, diatom analysis, archaeological site testing, sedimentary exposure sampling, and radiocarbon dating to construct sea level histories for the Hakai Passage region. Our data include 106 newly reported radiocarbon ages from key coastal sites that together support the thesis that this area has experienced a relatively stable sea level over the last 15,000 calendar years. These findings are significant in that they indicate a relatively stable coastal environment amenable to long-term human occupation and settlement of the area. Our results will help inform future archaeological investigations in the region

3500 Years of Shellfish Mariculture on the Northwest Coast of North America

Ancient systems of mariculture were foundations of social-ecological systems of many coastal Indigenous Peoples. However, since such systems either do not leave tangible remains in the archaeological record, and/or are hard to date, we know little about their development and use. Clam gardens, traditional mariculture features located within the intertidal zone along the Northwest Coast of North America, are composed of a rock wall positioned at the low tide mark and a flattened terrace on the landward side of the wall. Because these features are largely composed of rock and sediment, and have complex formation histories, they can be difficult to age. On northern Quadra Island, British Columbia, we identify three variations in clam garden form, constructed in different geomorphological settings, each of which require different sampling approaches to obtain ages on construction and ongoing use. To age the clam gardens, we consider radiocarbon dating of invertebrates that inhabit beach deposits (both pre- and post-garden construction), and the relationship of the gardens and clam samples to the local sea level history and taphonomic processes. Within our study area, we find clam gardens have been in use for 3500 years, likely corresponding to other social and ecological changes of the time. These data allow us to formulate guidelines on samples most suitable to constrain the age of initial and on-going wall construction and use of clam gardens, which can be extrapolated to dating other ancient mariculture features in other regions. Such dating programs are the foundation for understanding the long-term development of traditional marine management practices and how they are situated in broader social-ecological systems

Human occupation of northern India spans the Toba super-eruption ~74,000 years ago

Abstract: India is located at a critical geographic crossroads for understanding the dispersal of Homo sapiens out of Africa and into Asia and Oceania. Here we report evidence for long-term human occupation, spanning the last ~80 thousand years, at the site of Dhaba in the Middle Son River Valley of Central India. An unchanging stone tool industry is found at Dhaba spanning the Toba eruption of ~74 ka (i.e., the Youngest Toba Tuff, YTT) bracketed between ages of 79.6 ± 3.2 and 65.2 ± 3.1 ka, with the introduction of microlithic technology ~48 ka. The lithic industry from Dhaba strongly resembles stone tool assemblages from the African Middle Stone Age (MSA) and Arabia, and the earliest artefacts from Australia, suggesting that it is likely the product of Homo sapiens as they dispersed eastward out of Africa

Human occupation of northern India spans the Toba super-eruption ~74,000 years ago

Abstract: India is located at a critical geographic crossroads for understanding the dispersal of Homo sapiens out of Africa and into Asia and Oceania. Here we report evidence for long-term human occupation, spanning the last ~80 thousand years, at the site of Dhaba in the Middle Son River Valley of Central India. An unchanging stone tool industry is found at Dhaba spanning the Toba eruption of ~74 ka (i.e., the Youngest Toba Tuff, YTT) bracketed between ages of 79.6 ± 3.2 and 65.2 ± 3.1 ka, with the introduction of microlithic technology ~48 ka. The lithic industry from Dhaba strongly resembles stone tool assemblages from the African Middle Stone Age (MSA) and Arabia, and the earliest artefacts from Australia, suggesting that it is likely the product of Homo sapiens as they dispersed eastward out of Africa

Relationships between the macroscale sedimentology and micromorphology of glacigenic diamictons in south-central British Columbia

This research explores relationships between the macroscale sedimentology and micromorphology of glacigenic diamictons associated with the last Cordilleran Ice Sheet in south-central B.C., and tests the hypothesis that diamictons classified using traditional definitions and sedimentological criteria have unique micromorphologies. Cluster analyses suggest that diamicton types cannot be differentiated by the relative abundances of particular microstructures. Qualitative analyses suggest that microfabrics may be less consistent with macrofabrics in deformation tills and gravity flow diamictons than in undeformed or partially deformed lodgement tills, and evidence of water sorting at the microscale may be more prevalent in gravity flow diamictons than in primary tills. This suggests that though microstructures and microfabrics are not diagnostic to any one diamicton type, depositional processes express themselves at the microscale. Therefore diamicton micromorphology may be used to elaborate on diamicton depositional mechanics and post-depositional modification; however, interpretations are best made within the context of macroscale sedimentological inferences

Luminescence investigations into the time of final deposition of Toba volcanic ash and artefact-bearing alluvial sediments in the Middle Son Valley, Madhya Pradesh, India

Terraced alluvial deposits in the Middle Son Valley, Madhya Pradesh, India contain Youngest Toba Tuff (YTT) deposits and an archaeological record that spans the Acheulean to the Neolithic. For the past three decades, this region has been the focus of geological and archaeological investigations that aim to understand the impact of the ~74 thousand year (ka) Toba volcanic super-eruption on the environment and human populations in India. The research presented in this study is focussed on two main themes: 1) developing and applying luminescence dating techniques to alluvial sediments in the Middle Son Valley to assess the reliability of the YTT ash as a reliable chronostratigraphic marker in palaeoenvironmental investigations; and 2) to test a previously published model of alluvial deposition for the Middle Son Valley near the confluence of the Rehi and Son Rivers. The luminescence dating potential of potassium feldspar (KF) was explored at both the single aliquot and the single grain level for sediments in the Middle Son Valley. In this study, KF grains are shown to be suited to single-aliquot regenerative-dose measurement procedures and individual KF grains exhibit fading rates ranging from 0 to more than 20 %/decade. Post-infrared infrared signals (pIRIR) are shown to be less susceptible to anomalous fading, as expected, but evidence presented here suggests that pIRIR ages for alluvial sediments in the Middle Son Valley are less reliable than IRSL ages, because the source traps for these signals are less likely to be completely emptied by sun exposure during transport and deposition in the Son River. IRSL ages from KF and OSL ages from quartz presented in this study suggest that the final deposition of the sediments above and below the YTT ash deposits in the Middle Son Valley occurred sometime (possibly up to a few tens of thousands of years) after the Toba volcanic super-eruption. This chronology suggests that: 1) the YTT ash has been reworked by fluvial processes and cannot be considered a reliable chronostratigraphic marker as was previously thought; or 2) the YTT ash was deposited soon after the volcanic event ~74 ka ago, but the underlying sediments have since been eroded and replaced by younger, inset fluvial sediments. In both cases, the temporal framework presented here calls into question the validity of previous palaeoenvironmental interpretations and hypotheses that were based on pedogenic carbonates sampled above, below and within the YTT ash. A previously published model of alluvial deposition for the Middle Son Valley subdivides the preserved alluvium within the valley system into five stratigraphic formations. These formations include (from oldest to youngest) the Sihawal, Khunteli, Patpara, Baghor and Khetaunhi Formations. The accuracy of this model was tested near the Rehi-Son confluence using cross-valley topographic profiles, field observations, and IRSL age estimates from terraced alluvial sediments. IRSL age estimates for the highest terrace mark the beginning of incision of the Middle Son Valley alluvium at ~16 ka (as predicted by the model) and the termination of deposition of the Baghor Formation fine member silts between ~16 and ~21 ka ago. Maximum IRSL age estimates of ~1.9–2.7 ka, ~10 m above river level, mark the termination of deposition of the Khetaunhi Formation silts and the sands on the lowest terrace. These ages are only slightly younger than previously reported radiocarbon age estimates of ~3–5 ka for this formation. The age estimates for coarse sands and gravels that lie at intermediate elevations (~20 m above river level) between these two terraces contradict what is predicted by the model. According to the model, these sediments should be between ~40 and 58 ka in age and form part of the Patpara Formation, which has been exposed by fluvial erosion of the overlying Baghor Formation. By contrast, maximum IRSL age estimates presented here suggest that these deposits are only up to ~5–7 ka in age and form inset sediments that were deposited during a brief aggradational phase, sometime after incision of the highest alluvial surface ~16 ka ago. Incision of the Middle Son Valley alluvium across from the Rehi-Son confluence began shortly after ~16–21 ka, probably as a result of SW monsoon intensification. The inset coarse sand and gravel at ~20 m above river level likely aggraded under wet conditions in the early Holocene, and the lowest (~10 m high) inset terrace probably aggraded under more arid conditions during the late Holocene. Both the Middle Son Valley, and its neighbour, the Belan Valley, record major phases in regional climate and human occupation of northern India. Improved chronological control on the stratigraphies of these valleys therefore has implications for enhancing our understanding the history of climate change and human occupation in India

Assessing the time of final deposition of Youngest Toba Tuff deposits in the Middle Son Valley, northern India

We present optical ages for quartz and K-feldspar grains extracted from sedimentary deposits above and below Youngest Toba Tuff (YTT) at two localities in the Middle Son Valley (MSV), Madhya Pradesh, India. These ash deposits have been the focus of past palaeoenvironmental investigations that aim to understand the effects of the ~ 74 ka Toba super-eruption on ecosystems and human populations in northern India. Age estimates from both quartz and feldspar grains post-date the Toba eruption and single-grain age distributions suggest that YTT ash-bearing sediments in the MSV are mainly composed of a mixture of recently sun-exposed, flood-transported grains and older grains from slumped river bank deposits. This study highlights the hazards of using YTT ash as an isochronous marker horizon in the MSV, and illustrates the need to independently assess the time of final deposition of reworked volcanic ash beds in sedimentary sequences used in stratigraphic correlations and palaeoenvironmental reconstructions

Testing a model of alluvial deposition in the Middle Son Valley, Madhya Pradesh, India - IRSL dating of terraced alluvial sediments and implications for archaeological surveys and palaeoclimatic reconstructions

Over the past three decades, the Middle Son Valley, Madhya Pradesh, India has been the focus of archaeological, geological, and palaeoenvironmental investigations that aim to reconstruct regional climate changes in the Late Pleistocene and to understand the effects of the ∼74 ka Toba super-eruption on ecosystems and human populations in northern India. The most recently published model of alluvial deposition for the Middle Son Valley subdivides its alluvium into five stratigraphic formations, each associated with a specific artefact assemblage. In this study, new cross-valley topographic profiles, field observations and infrared stimulated luminescence (IRSL) age estimates are used to refine this model south of the Rehi-Son River confluence. These data not only provide insights into the fluvial history of the Son River and its response to changes in palaeoclimate, but will also inform future archaeological surveys by constraining the geomorphic context of surficial and excavated artefacts in the area