Evidence for a Recent Formation of the Isthmus of Panama 0.6 Mya

The exact age of the final formation of the Isthmus of Panama is a critical reference point for oceanographic, climatic, biogeographic, and evolutionary hypotheses. Geotectonic evidence suggests that the isthmus was completed between 12 Mya and 3 Mya, and an age of 3–4 Mya has been used as a benchmark in hundreds of studies over the past 30 years. Phylogeographic data indicate the existence of marine connections across the isthmus much more recently, however. I reconsider the available geotectonic, biostratigraphic, oceanographic, and paleoclimatic data and show that multiple lines of indirect evidence suggest that four transisthmian seaways may have persisted until as recently as the onset of the Middle Pleistocene (~ 0.6 Mya). Subduction of the Cocos Plate beneath one transisthmian seaway (the only seaway featuring a deep sill) caused rapid tectonic shoaling and reorganisation of oceanic currents, which coincided with a major glacioeustatic sea level fall ~ 950–917 Mya that led to a temporary closing of the Bering Strait. This resulted in unusual and contrasting climate phenomena, including the “900-Kyr (cold) event” and the “greening” of South Greenland during the MIS 22 glacial maximum. The concurrence of the final formation of the Isthmus of Panama with the mid-Pleistocene Transition of glacial/interglacial periodicity suggests a tight relationship between these events.

A harmonically weighted filter for cyclical long memory processes

The estimation of the long memory parameter d is a widely discussed issue in the literature. The harmonically weighted (HW) process was recently introduced for long memory time series with an unbounded spectral density at the origin. In contrast to the most famous fractionally integrated process, the HW approach does not require the estimation of the d parameter, but it may be just as able to capture long memory as the fractionally integrated model, if the sample size is not too large. Our contribution is a generalization of the HW model, denominated the Generalized harmonically weighted (GHW) process, which allows for an unbounded spectral density at $$k \ge 1$$ k ≥ 1 frequencies away from the origin. The convergence in probability of the Whittle estimator is provided for the GHW process, along with a discussion on simulation methods. Fit and forecast performances are evaluated via an empirical application on paleoclimatic data. Our main conclusion is that the above generalization is able to model long memory, as well as its classical competitor, the fractionally differenced Gegenbauer process, does. In addition, the GHW process does not require the estimation of the memory parameter, simplifying the issue of how to disentangle long memory from a (moderately persistent) short memory component. This leads to a clear advantage of our formulation over the fractional long memory approach.

Paleoclimatic inferences for the Holocene of southern Brazil in environments influenced by different hydrological systems

Organic matter preserved in continental deposits is an excellent archive of environmental and climate change, and various research tools are commonly used to access this information. The present study is the result of a study conducted in continental environments influenced by different hydrological systems and demonstrates the usefulness of these analyses in obtaining rainfall-related paleoclimatic data. The study area is located in the basalts of the Alto Uruguai region (Rio Grande do Sul, Brazil), known for its mineral riches such as amethyst geodes and hot springs. The methodology used integrates palynofacies and organic geochemistry techniques, complemented by radiocarbon dating. The results were obtained from three sedimentary profiles and showed a trend of relative decrease in rainfall levels in the region. However, the scale of these changes is also highly influenced by the hydrological system of the depositional environment that originated the sedimentary record at each site. Based on the data obtained, we advise the establishment of medium- and long-term policies involving better management of local natural resources in the investigated region.

Asymptotic analysis of internal relaxation oscillations in a conceptual climate model

We construct a dynamical system based on the Källén–Crafoord–Ghil conceptual climate model which includes the ice–albedo and precipitation–temperature feedbacks. Further, we classify the stability of various critical points of the system and identify a parameter which change generates a Hopf bifurcation. This gives rise to a stable limit cycle around a physically interesting critical point. Moreover, it follows from the general theory that the periodic orbit exhibits relaxation-oscillations that are a characteristic feature of the Pleistocene ice ages. We provide an asymptotic analysis of their behaviour and derive a formula for the period along with several estimates. They, in turn, are in a decent agreement with paleoclimatic data and are independent of any parametrization used. Whence, our simple but robust model shows that a climate may exhibit internal relaxation oscillations without any external forcing and for a wide range of parameters.

In search of a Paleolithic Silk Road in Kazakhstan

Paleoanthropological data suggest that the Late Pleistocene was a time of population contact and possibly dispersal in Central Asia. Geographic and paleoclimatic data suggest that a natural corridor through Kazakhstan linked areas to the north and east (Siberia, China) to those further to the west and south (Uzbekistan), much akin to a Paleolithic Silk Road. We review the known Pleistocene archaeology and paleoclimatic setting of this region and provide a geoarchaeological framework for contextualizing preliminary survey results of the PALAEOSILKROAD project’s first three seasons of fieldwork. We discuss some systematic biases in three geomorphic and sedimentary archives: karst, loess, and spring deposits, specifying ways in which these biases might determine the kinds of data that are extractable by systematic survey. In particular, we caution about the possibility of future systematic biases in chronology that could come about as a result of the type of geomorphic context in which the sites are recovered. We conclude with recommendations for future work in the area.

Asynchronous trilobite extinctions at the early to middle Cambrian transition

Trilobites appeared and diversified rapidly in the Cambrian, but it is debated as to whether their radiations and extinctions were globally synchronous or geographically restricted and diachronous. The end of the early Cambrian is a classic example—it has traditionally been defined by the extinction of olenellid and redlichiid trilobites and the appearance of paradoxidid trilobites. Here we integrate the global biostratigraphy of these three trilobite groups with high-precision tuff and tandem detrital zircon U-Pb age constraints to falsify prior models for global synchronicity of these events. For the first time, we demonstrate that olenellid trilobites in Laurentia went extinct at least 3 Ma after the first appearance of paradoxidids in Avalonia and West Gondwana (ca. 509 Ma). They also disappeared before the extinction of redlichiids and prior to the base of the Miaolingian at ca. 506 Ma in South China. This indicates that these three trilobite groups (paradoxidids, olenellids, and redlichiids) and their associated biotas overlapped in time for nearly 40% of Cambrian Epoch 2, Age 4. Implications of this chronological overlap are: (1) trilobite transitions were progressive and geographically mediated rather than globally synchronous; and (2) paleontological databases underestimate the diversity of the early Cambrian. This ∼3 Ma diachroneity, at a critical time in the early evolution of animals, also impacts chemostratigraphic and paleoclimatic data sets that are tied to trilobite biostratigraphy and that collectively underpin our understanding of the Cambrian Earth system.

Climate

This chapter serves as an introduction to and reference for climate–human behavior studies in the Southwest. These studies investigate potential climatic impacts on social change and historical trajectories. To build foundational understanding, a representative climate–human behavior model is presented and evaluated, commonly used paleoclimatic data are detailed, and methods for identifying climate extremes (e.g., droughts, wet periods) in these data are described. Some extreme climate events and the challenge of identifying their influence (if any) on social change are noted. A familiarity with these aspects of climate–human behavior studies is essential for effectively evaluating interpretations of historical trajectories that invoke climatic influences.