The "African humid period" and the record of marine upwelling from excess ^(230)Th in Ocean Drilling Program Hole 658C

Using a high-resolution ^(230)Th normalized record of sediment flux, we document the deglacial and Holocene history of North African aridity and coastal upwelling at Ocean Drilling Program Hole 658C. At both the end of the Younger Dryas and after the 8.2 ka event, there are significant drops in terrigenous accumulation at our site, indicating an increase in the monsoon moisture flux over Africa at this time. At 5.5 ka, there is an abrupt end to the “African humid period” and a return to stronger upwelling conditions. For carbonate and opal fluxes the ^(230)Th normalization completely changes the shape of each record based on percentage variations alone. This site is a clear example of how variations in one sediment component can obscure changes in the others, and it demonstrates the need for radionuclide measurements more generally in paleoceanography. By taking our new records and a large amount of previous data from this site we conclude that increases in African moisture are tightly coupled to decreases in coastal upwelling intensity

Cultural Responses to Climate Change During the Late Holocene

This study describes the effect of drought on past civilizations and peoples and their response as civilizations or cultures. The site includes four case studies drawn from New and Old World civilizations that document societal responses to prolonged drought, including population dislocations, urban abandonment, and state collapse. Further study of past cultural adaptations to persistent climate change may provide valuable perspective on possible responses of modern societies to future climate change. Educational levels: High school, Undergraduate lower division

High- and low-latitude climate interactions: evidence for enhanced aridity of Asian monsoon dust source areas after 2.4 MYR from ODP Leg 117 magnetic susceptibility data

EXTRACT (SEE PDF FOR FULL ABSTRACT): Whole-core magnetic susceptibility can sometimes be used as a rapid and sensitive indicator of variations in the concentration of terrigenous material. We apply this approach to study the evolution of Plio-Pleistocene climatic cycles of terrigenous sedimentation at Ocean Drilling Program Site 721, on the Owen Ridge in the Arabian Sea

Depth of post-depositional remanence acquisition in deep-sea sediments: a case study of the Brunhes-Matuyama reversal and oxygen isotopic Stage 19.1

Although post-depositional remanent magnetization (PDRM) in deep-sea sediments appears to be acquired during the earliest stages of sediment compaction, the natural variability of the PDRM lock-in depth in deep-sea sediments is poorly understood and as yet unquantified. Here we consider variations in the relative stratigraphic positions of oxygen isotopic interglacial Stage 19.1 and the Brunhes-Matuyama (B/M) Chronozone reversal for eight deep-sea sediment cores. Results from a similar study of the displacement between a widespread microtektite layer and the B/M boundary are also included [1]. The PDRM lock-in depth and the temporal relationships between the B/M and Stage 19.1 datums can be determined from the offsets between the paleomagnetic and the isotopic (and microtektite) stratigraphies. For cores with sedimentation rates greater than 1 cm kyr−1, the depth offset between the paleomagnetic and isotopic datums is a linear function of sedimentation rate. A simple model (r2 = 0.77) demonstrates that (1) PDRM acquisition occurs ∼ 16 cm below the sediment surface, and (2) the B/M reversal occurs 6 kyr (±2 kyr) after the Stage 19.1 datum, and the Stage 19.1 datum occurs 9 kyr (±3 kyr) after the deposition of the Australasian microtektite strewnfield. This example argues against simple geophysical models linking geomagnetic field reversals to climate change or impact events. The B/M boundary is anomalously deep (30–50 cm) in very low accumulation rate sediments ( < 1 cm kyr−1) and this may reflect the unusual physical properties of these sediments. A review of the geotechnical literature suggests that very low accumulation rate sediments have sufficient time to develop enhanced interparticle rigidity (structural strength) which inhibits early compaction and, hence, PDRM acquisition

Changes in biological productivity along the northwest African margin over the past 20,000 years

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 31 (2016): 185–202, doi:10.1002/2015PA002862.The intertropical convergence zone and the African monsoon system are highly sensitive to climate forcing at orbital and millennial timescales. Both systems influence the strength and direction of the trade winds along northwest Africa and thus directly impact coastal upwelling. Sediment cores from the northwest African margin record upwelling-related changes in biological productivity connected to changes in regional and hemispheric climate. We present records of 230Th-normalized biogenic opal and Corg fluxes using a meridional transect of four cores from 19°N–31°N along the northwest African margin to examine changes in paleoproductivity since the last glacial maximum. We find large changes in biogenic fluxes synchronous with changes in eolian fluxes calculated using end-member modeling, suggesting that paleoproductivity and dust fluxes were strongly coupled, likely linked by changes in wind strength. Opal and Corg fluxes increase at all sites during Heinrich Stadial 1 and the Younger Dryas, consistent with an overall intensification of the trade winds, and changes in the meridional flux gradient indicate a southward wind shift at these times. Biogenic fluxes were lowest, and the meridional flux gradients were weakest during the African Humid Period when the monsoon was invigorated due to precessional changes, with greater rainfall and weaker trade winds over northwest Africa. These results expand the spatial coverage of previous paleoproxy studies showing similar changes, and they provide support for modeling studies showing changes in wind strength and direction consistent with increased upwelling during abrupt coolings and decreased upwelling during the African Humid Period.NSF Grant Numbers: OCE-1103262, OCE-1030784, OCE-0402348; Center for Climate and Life2016-07-2

Complex Network Approach for Recurrence Analysis of Time Series

We propose a novel approach for analysing time series using complex network theory. We identify the recurrence matrix calculated from time series with the adjacency matrix of a complex network, and apply measures for the characterisation of complex networks to this recurrence matrix. By using the logistic map, we illustrate the potentials of these complex network measures for detecting dynamical transitions. Finally we apply the proposed approach to a marine palaeo-climate record and identify subtle changes of the climate regime.Comment: 23 pages, 7 figure

Climate Shocks

Changes in climate are emerging as elements that shaped human evolution over millions of years, as scientists learn that such alteration coincided with the extinction of some of our ancestors and the success of others. Evidence from ancient soils in East Africa, deep-sea sediments and fossil teeth from our forerunners combines to reveal rapid swings between wet and dry environments, as well two distinct periods when grasslands replaced more wooded areas. The emergence of our own genus, Homo, our varied diet, advances in stone tool technology and the very human trait of adaptability in the face of ongoing change may be tied to these episodes, according to one theory

Sporadic sampling, not climatic forcing, drives observed early hominin diversity.

Paleoanthropologists have long been intrigued by the observed patterns of human evolution, including species diversity, and often invoked climatic change as the principal driver of evolutionary change. Here, we investigate whether the early hominin fossil record is of suitable quality to test these climate-forcing hypotheses. Specifically, we compare early hominin diversity to sampling metrics that quantify changes in fossil preservation and sampling intensity between 7 and 1 million years ago. We find that observed diversity patterns are governed by sporadic sampling and do not yield a genuine evolutionary signal. Many more fossil discoveries are required before existing hypotheses linking climate and evolution can be meaningfully tested.The role of climate change in the origin and diversification of early hominins is hotly debated. Most accounts of early hominin evolution link observed fluctuations in species diversity to directional shifts in climate or periods of intense climatic instability. None of these hypotheses, however, have tested whether observed diversity patterns are distorted by variation in the quality of the hominin fossil record. Here, we present a detailed examination of early hominin diversity dynamics, including both taxic and phylogenetically corrected diversity estimates. Unlike past studies, we compare these estimates to sampling metrics for rock availability (hominin-, primate-, and mammal-bearing formations) and collection effort, to assess the geological and anthropogenic controls on the sampling of the early hominin fossil record. Taxic diversity, primate-bearing formations, and collection effort show strong positive correlations, demonstrating that observed patterns of early hominin taxic diversity can be explained by temporal heterogeneity in fossil sampling rather than genuine evolutionary processes. Peak taxic diversity at 1.9 million years ago (Ma) is a sampling artifact, reflecting merely maximal rock availability and collection effort. In contrast, phylogenetic diversity estimates imply peak diversity at 2.4 Ma and show little relation to sampling metrics. We find that apparent relationships between early hominin diversity and indicators of climatic instability are, in fact, driven largely by variation in suitable rock exposure and collection effort. Our results suggest that significant improvements in the quality of the fossil record are required before the role of climate in hominin evolution can be reliably determined