Late Pleistocene exploration and settlement of the Americas by modern humans

American Association for the Advancement of Science (AAAS) | June 12th, 2019

North and South America were the last continents to be explored and settled by modern humans at the end of the Pleistocene. Genetic data, derived from contemporary popul

North and South America were the last continents to be explored and settled by modern humans at the end of the Pleistocene. Genetic data, derived from contemporary populations and ancient individuals, show that the first Americans originated from Asia and after several population splits moved south of the continental ice sheets that covered Canada sometime between ~17.5 and ~14.6 thousand years (ka) ago. Archaeological evidence shows that geographically dispersed populations lived successfully, using biface, blade, and osseous technologies, in multiple places in North and South America between ~15.5 and ~14 ka ago. Regional archaeological complexes emerged by at least ~13 ka ago in North America and ~12.9 ka ago in South America. Current genetic and archaeological data do not support an earlier (pre–17.5 ka ago) occupation of the Americas.

Levee Failures in the Sacramento-San Joaquin River Delta: Characteristics and Perspectives

Texas A&M University | December 15th, 2011

Between 1850 and 1922, agriculturalists built 1,700 kilometers of levees to convert 250,000 hectares of tidal marsh to farmland where the San Joaquin and Sacramento River

Between 1850 and 1922, agriculturalists built 1,700 kilometers of levees to convert 250,000 hectares of tidal marsh to farmland where the San Joaquin and Sacramento Rivers enter the San Francisco Bay (the Delta). Drained, farmed and isolated from the water channels, the organic soils behind the levees subsided to elevations as low as 8 meters below sea level, turning "levees" into "dams" that hold back water constantly. Engineers built water transfer projects in the mid-20th century, transferring water from the south Delta to 25 million Californians who now rely on the "dams" accidentally converted into supply channels. In 1972, however, a levee failure caused a salt-water intrusion into the Delta, raising the prominence of the polemic Peripheral Canal which, if built would replace the levees in the trans-Delta water transport role. Levee failures in 2004 (the Delta) and 2005 (New Orleans) have re-ignited the debate, fueled by comments made by public officials who warned that the Delta levees posed more risk of failure than did the pre-Katina Louisiana levees. This background motivates two research questions: What are the social perspectives regarding levee failures of the experts managing the Delta; and what is the history of levee failures that might support their perspectives? The research employed Q-Method to identify and describe four social perspectives: Delta Sustainers, Abandon the Levees, Levee Pragmatists, and Multi-Purpose Levee Advocates. A critical element underlying differences among the perspectives revolved around the perceived history of failures of Delta levees. This dissertation employed semi-structured interviews, archival record searches, and historic map and aerial photograph comparisons to compile a history of 265 levee failures since 1868, many of which are referenced to location, segment, and levee type. In addition, the dissertation compiled a list of emergency repairs and successful flood-fights. The history of failures indicates that important levees of the Delta have performed significantly better than previously identified. Sharing these social perspectives and research results among the key actors addressing Delta issues may lead to improved consensus decisions.

Little Ice Age flood events recorded in sag pond sediments in the Carrizo Plains National Monument, California

Nature Portfolio (Springer Nature) | March 8th, 2024

In California, severe precipitation events (SPEs) are often associated with winter season atmospheric rivers. These SPEs can generate hurricane-scale precipitation, creat

In California, severe precipitation events (SPEs) are often associated with winter season atmospheric rivers. These SPEs can generate hurricane-scale precipitation, creating a variety of natural hazards such as floods and landslides. For California, the most complete SPE-flood record yet generated is a 9000-year paleoflood reconstruction from the Santa Barbara Basin (Du et al. in Mar Geol 397:29–42, 2018). Finding terrestrial counterparts to compare to the Santa Barbara Basin is a challenge in Southern California where lake basins are rare and the terrestrial spatiotemporal signature of SPE floods remains largely unconstrained. Here, we present five 1–2 m sediment cores from a sag pond along the San Andreas Fault in the southern Carrizo Plain. The most complete record (core CLPC21-4) was selected for a variety of chronological, sedimentological, and biological analyses. Principal Component Analysis on these data reveals two end member sediment unit types: event versus ambient sedimentation. We focus on the two thickest sediment units likely generated by SPEs. These two units (EU 1 and EU 2) are especially distinct within CLPC21-4, characterized by erosive bases, above average sand content, normal grading, low magnetic susceptibility, and low total organic matter. Moreover, they are visually apparent across all five cores within the sag pond and thin from source to sink. Age control for the two units is constrained by AMS 14C dates on discrete organic materials and supported by the identification of Erodium’s first appearance ca. 1750–1765 CE in the Santa Barbara region. Using these age constraints, we infer the maximum limiting age range for Event Unit 1 from 1470 to 1640 CE and Event Unit 2 from 1740 to 1800 CE. Within the limits of dating, site-specific proxy sensitivities, and inherent meteorological heterogeneity, we propose a probable correlation to Santa Barbara Basin flood events at 1525 CE and 1760 CE. Our results suggest that sag ponds may represent a viable and untapped paleoclimatic archive for California. Future work will focus on a latitudinal series of sag ponds to determine the spatiotemporal sequence and correlativity of SPEs in the sediment record.

Memorialization and Memory of Southern California's St. Francis Dam Disaster of 1928

California State University, Northridge (CSUN) | August 1st, 2014

The commemoration of disasters is a product of social, cultural, economic, and  political forces in human society. Southern California's largely unheard-of St. Francis D

The commemoration of disasters is a product of social, cultural, economic, and  political forces in human society. Southern California's largely unheard-of St. Francis Dam Disaster of 1928 provides an excellent opportunity to study this complex process of commemoration, engaging memory within difference frames of reference. In particular, evaluating how and why this man-made dam disaster has been forgotten on a state and national level, but tenuously remembered within the flood-zone, allows for consideration of the diversity of commemorative processes in the construction of memory and heritage related to major catastrophes.

Mission Bay Historical Ecology Reconnaissance Study: Data Collection Summary (Technical Report)

San Francisco Estuary Institute (SFEI) | February 1st, 2016

The goals of the Mission Bay Historical Ecology Reconnaissance Study were to collect and compile high-priority historical data about the Mission Bay landscape, identify

The goals of the Mission Bay Historical Ecology Reconnaissance Study were to collect and compile high-priority historical data about the Mission Bay landscape, identify sources that could help to develop a deeper understanding of early ecological conditions, and to identify future possible research directions based on the available data. This technical memorandum is intended to document the archives consulted during the reconnaissance study, summarize the collected and compiled data, and to identify potential next steps. A separate technical presentation to project staff and advisors will summarize the preliminary findings and questions generated from a review of the historical dataset. Ultimately, this research is intended to support the San Diego Audubon Society’s Mission Bay Wetlands Conceptual Restoration Plan (CRP) and the ReWild Mission Bay project. Also on this topic: Mission Bay Historical Ecology Reconnaissance Study: Data CollectionSummary and Initial Findings (Presentation)

Natural Curiosities of California

Scientific American | April 12th, 1862

Necessity for Comprehensive Adjudication of Water Rights on the Sacramento and San Joaquin Rivers in Aid of the Central Valley Project

U.S. Government Publishing Office (GPO) | November 30th, 1942

North American drought: Reconstructions, causes, and consequences

Earth Science Reviews (Elsevier) | January 3rd, 2007

Severe drought is the greatest recurring natural disaster to strike North America. A remarkable network of centuries-long annual tree-ring chronologies has now allowed fo

Severe drought is the greatest recurring natural disaster to strike North America. A remarkable network of centuries-long annual tree-ring chronologies has now allowed for the reconstruction of past drought over North America covering the past 1000 or more years in most regions. These reconstructions reveal the occurrence of past “megadroughts” of unprecedented severity and duration, ones that have never been experienced by modern societies in North America. There is strong archaeological evidence for the destabilizing influence of these past droughts on advanced agricultural societies, examples that should resonate today given the increasing vulnerability of modern water-based systems to relatively short-term droughts. Understanding how these megadroughts develop and persist is a timely scientific problem. Very recently, climate models have succeeded in simulating all of the major droughts over North America from the Civil War to the severe 1998–2004 drought in the western U.S. These numerical experiments indicate the dominating importance of tropical Pacific Ocean sea surface temperatures (SSTs) in determining how much precipitation falls over large parts of North America. Of central importance to drought formation is the development of cool “La Niña-like” SSTs in the eastern tropical Pacific region. This development appears to be partially linked to changes in radiative forcing over that region, which affects the Bjerknes feedback mechanism of the ENSO cycle there. Paradoxically, warmer conditions over the tropical Pacific region lead to the development of cool La Niña-like SSTs there, which is drought inducing over North America. Whether or not this process will lead to a greater prevalence of drought in the future as the world warms due to accumulating greenhouse gases is unclear at this time.

North American Droughts of the Last Millennium from a Gridded Network of Tree-Ring Data

American Meteorological Society (AMS) | April 1st, 2007

Drought is the most economically expensive recurring natural disaster to strike North America in modern times. Recently available gridded drought reconstructions

Drought is the most economically expensive recurring natural disaster to strike North America in modern times. Recently available gridded drought reconstructions have been developed for most of North America from a network of drought-sensitive tree-ring chronologies, many of which span the last 1000 yr. These reconstructions enable the authors to put the famous droughts of the instrumental record (i.e., the 1930s Dust Bowl and the 1950s Southwest droughts) into the context of 1000 yr of natural drought variability on the continent. We can now, with this remarkable new record, examine the severity, persistence, spatial signatures, and frequencies of drought variability over the past milllennium, and how these have changed with time. The gridded drought reconstructions reveal the existence of successive “megadroughts,” unprecedented in persistence (20–40 yr), yet similar in year-to-year severity and spatial distribution to the major droughts experienced in today’s North America. These megadroughts occurred during a 400-yr-long period in the early to middle second millennium A.D., with a climate varying as today’s, but around a drier mean. The implication is that the mechanism forcing persistent drought in the West and the Plains in the instrumental era is analagous to that underlying the megadroughts of the medieval period. The leading spatial mode of drought variability in the recontructions resembles the North American ENSO pattern: widespread drought across the United States, centered on the Southwest, with a hint of the opposite phase in the Pacific Northwest. Recently, climate models forced by the observed history of tropical Pacific SSTs have been able to successfully simulate all of the major North American droughts of the last 150 yr. In each case, cool “La Niña–like” conditions in the tropical Pacific are consistent with North American drought. With ENSO showing a pronounced signal in the gridded drought recontructions of the last millennium, both in terms of its link to the leading spatial mode, and the leading time scales of drought variability (revealed by multitaper spectral analysis and wavelet analysis), it is postulated that, as for the modern day, the medieval mega- droughts were forced by protracted La Niña–like tropical Pacific SSTs. Further evidence for this comes from the global hydroclimatic “footprint” of the medieval era revealed by existing paleoclimatic archives from the tropical Pacific and ENSO-sensitive tropical and extratropical land regions. In general, this global pattern matches that observed for modern-day persistent North American drought, whereby a La Niña–like tropical Pacific is accompanied by hemispheric, and in the midlatitudes, zonal, symmetry of hydroclimatic anomalies.

North American droughts of the mid to late nineteenth century: a history, simulation and implication for Mediaeval drought

Holocene (Sage Journals) | February 1st, 2006

Unlike the major droughts of the twentieth century that are readily identified in the instrumental record, similar events in the nineteenth century have to be identified

Unlike the major droughts of the twentieth century that are readily identified in the instrumental record, similar events in the nineteenth century have to be identified using a combination of proxy data, historical accounts and a sparse collection of early instrumental records. In the USA, three distinct periods of widespread and persistent drought stand out in these records for the latter half of the nineteenth century: 1856-1865, 1870-1877 and 1890-1896. Each of these events is shown to coincide with the existence of an anomalously cool, La Nifia-like tropical Pacific. To examine the physical mechanisms behind these droughts two ensembles of simulations with an atmosphere general circulation model (AGCM) were generated: the first forces an AGCM with the observed history of Sea Surface Temperatures (SSTs) everywhere from 1856 to 2001 (the GOGA experiment), the second forces the AGCM only with tropical Pacific SSTs, being coupled to a two-layer entraining mixed layer (ML) ocean elsewhere (the POGA-ML experiment). Owing to a sparsity of instrumental precipitation data at this time, proxy evidence from tree rings is used as verification. A comparison of modelled soil moisture with tree-ring reconstructions of the Palmer Drought Severity Index (PDSI), a proxy for soil moisture, from the North American Drought Atlas is made. Both the POGA-ML and GOGA ensemble means capture the three multi-year droughts of the mid to late nineteenth century, indicating that the droughts were SST forced. The similarity of the POGA-ML and GOGA simulations implies that the component of each drought signal that is forced by the SST is driven ultimately by the La Nifia-like tropical Pacific. The global atmosphere-ocean context of each of the mid to late-nineteenth century droughts reveals a zonally and hemispherically symmetric pattern consistent with forcing from the tropics. In addition, Rossby wave propagation from the cooler equatorial Pacific amplifies dry conditions over the USA. Finally, using published coral data for the last millennium to reconstruct a NINO 3.4 history, the modern-day relationship between NINO 3.4 and North American drought is applied to recreate two of the severest Mediaeval ‘drought epochs’ in the western USA. The large-scale spatial similarity to the Drought Atlas data demonstrates the potential link between a colder eastern equatorial Pacific and the persistent North American droughts of the Mediaeval period.