Which of these is the best description of a paleoclimate proxy? group of answer choices

Source: http://www.globalchange.umich.edu/globalchange1/current/lectures/kling/paleoclimate/index.html

An important method for the study of long-term climate change involves isotope geochemistry. Oxygen is composed of 8 protons, and in its most common form with 8 neutrons, giving it an atomic weight of 16 (16O) -- this is know as a "light" oxygen. It is called "light" because a small fraction of oxygen atoms have 2 extra neutrons and a resulting atomic weight of 18 (18O), which is then known as "heavy" oxygen.  18O is a rare form and is found in only about 1 in 500 atoms of oxygen.

The ratio of these two oxygen isotopes has changed over the ages and these changes are a proxy to changing climate that have been used in both ice cores from glaciers and ice caps and cores of deep sea sediments.  Many ice cores and sediment cores have been drilled in Greenland, Antarctica and around the world's oceans. These cores are actively studied for information on variations in Earth's climate.

Climate Temperature from Ice Cores

Which of these is the best description of a paleoclimate proxy? group of answer choices
Figure 1. Light oxygen in water (H216O) evaporates more readily that water with heavy oxygen (H218O). Hence oceans will be relatively rich in 18O when glaciers grow and hold the precipitated 16O.

Ice in glaciers has less 18O than the seawater, but the proportion of heavy oxygen also changes with temperature. To understand why this might be so, we need to think about the process of glacier formation. The water-ice in glaciers originally came from the oceans as vapor, later falling as snow and becoming compacted in ice. When water evaporates, the heavy water (H218O) is left behind and the water vapor is enriched in light water (H216O). This is simply because it is harder for the heavier molecules to overcome the barriers to evaporation. Thus, glaciers are relatively enhanced in 16O, while the oceans are relatively enriched in 18O. This imbalance is more marked for colder climates than for warmer climates. In fact, it has been shown that a decrease of one part per million 18O in ice reflects a 1.5°C drop in air temperature at the time it originally evaporated from the oceans.

While there are complexities with the analysis, a simple measurement of the isotopic ratio of 18O in ice cores can be directly related to climate. Ice cores from Greenland or Antarctica are often layered, and the layers can be counted to determine age. The heavy oxygen ratio can then be used as a thermometer of ancient climates.

 

Global Ocean-Ice Water Budget

Source: http://www.uvm.edu/~cmehrten/courses/earthhist/paleoclimind.ppt

H2O is evaporated from sea water. The oxygen in the H2O is enriched in the lighter 16O. This H2O condenses in clouds, falling on land as precipitation. Thus, H2O that is part of the terrestrial water cycle is enriched in the light 16O isotope and sea water is enriched in the heavier 18O isotope. Glacial ice is therefore made up primarily of water with the light 16O isotope. This leaves the oceans enriched in the heavier 18O, or “more positive.” During glacial periods, more 16O is trapped in glacial ice and the oceans become even more enriched in 18O. During interglacial periods, O16 melts out of ice and the oceans become less 18O rich, or “more negative” in 18O.

Which of these is the best description of a paleoclimate proxy? group of answer choices

Climate Temperatures from Ocean Sediments

Source: http://www.globalchange.umich.edu/globalchange1/current/lectures/kling/paleoclimate/index.html

The same isotopic analyses can be made in ocean sediment cores on the shells of dead marine organisms (Figure 6).  Some of these organisms are made up of calcium carbonate (CaCO3), and the oxygen in the carbonate reflects the isotopic abundance in the shallow waters where the creatures lived. Thus if we can find and date ever more ancient sediments made up of old sea shells, we can determine the isotopic ratio of oxygen and infer the sea surface temperature at that time. The more 18O found in the sediment, the colder the climate (inverse relationship to that of glacier ice).

Which of these is the best description of a paleoclimate proxy? group of answer choices

Figure 6. A micro-photograph of small skeleton-bearing plankton sea creatures. When such creatures die, their shells fall to the bottom of the ocean, carrying the tell-tail oxygen isotopic ratio appropriate to the temperature of the surface waters where they lived.

Which of these is the best description of a paleoclimate proxy? group of answer choices

The idea that critical junctures in human evolution and behavioral development may have been shaped by environmental factors has been around since Darwin. Although various hypotheses and models have been proposed, refined, and/or abandoned for at least a century, the concept of environmental determinism and hominin evolution is still a hot topic today. While it is ultimately local-level environmental processes acting upon individual populations that is one of the driving forces of evolutionary change, such shifts are often framed within the context of much larger regional or global climatic trends.

Long-Term Records of Paleoclimate

Direct measurements of climate components such as temperature and precipitation only exist for the last century or two. To reconstruct climate over longer time-scales, scientists indirectly measure these components by analyzing various proxies, or indicators, that are sensitive to climatic or environmental parameters and preserved in the geological record. Proxy records from marine sediment and ice cores provide the basis for much of our understanding of past climate. These long-term and relatively continuous natural archives are often used as references for comparison with local terrestrial-based paleoenvironmental reconstructions. For example, the record of oxygen and hydrogen isotope ratios preserved in glacial ice, and oxygen isotope ratios in the shells of marine organisms such as foraminifera and radiolaria, provide a record of past sea levels, ice volume, seawater temperature and global atmospheric temperature (Figures 1 & 2). Air bubbles trapped in ice cores also provide a direct record of the past chemical composition of the atmosphere, particularly CO2. Carbon isotope ratios of shells in marine cores are equally valuable for estimates of water circulation and atmospheric CO2 concentrations. Eolian dust preserved in both marine sediment and ice cores has been correlated with climate and environmental conditions in the dust's source region, specifically as a proxy for aridity. Continuous ice cores from Greenland record back to over 100,000 years ago (Bender et al. 2002), while those from Antarctica extend back to ~800,000 years ago (Lambert et al. 2008). Thus, these records are relevant to the later members of the genus Homo, such as H. erectus, H. heidelbergensis, H. neanderthalensis, and H. sapiens. Documenting a much longer timescale, marine sediment cores have been collected across the globe, and composite records have been compiled that extend beyond the Cenozoic, thus covering the entire duration of the Primate fossil record (Zachos et al. 2001).

Which of these is the best description of a paleoclimate proxy? group of answer choices

Figure 1: EPICA Dome C (EDC, Antarctica) data in comparison with other climatic indicators.

a, Stable isotope (δD) record from EDC. b, Vostok dust flux record. c, EDC dust flux records (numbers indicate Marine Isotope Stages). d, EDC dust size data expressed as fine particle percentage. e, Marine sediment δ18O stack (proxy for global ice volume). f, Magnetic susceptibility stack record for Chinese loess. Peaks in most records depicted and odd MIS numbers indicate interglacial phases while troughs and even MIS numbers indicate glacial phases.


Which of these is the best description of a paleoclimate proxy? group of answer choices

Figure 2: Global deep-sea oxygen and carbon isotope records.

Records for the Cenozoic based on data compiled from more than 40 DSDP and ODP sites with key climatic events indicated.

There are a variety of other important high-resolution paleoclimate records relevant to hominin evolutionary history, but these are temporally or spatially restricted compared to marine cores. For example, the variation in thickness and grain size in Chinese loess deposits are related to extensive periods of cold, dry, winter Asian monsoon winds stretching back over the last 7 million years (An 2000). Speleothems found in caves are also a rich archive of local paleoclimate information and, combined with uranium-thorium dating, can provide high-resolution records back to 500,000 years ago. Carbon and oxygen isotopic analysis as well as relative growth band thickness of speleothems have provided proxy data for local temperature, rainfall, aridity, and overlying vegetation (C3 vs. C4 plants) at hominin sites in South Africa, Europe, the Levant, and Asia (e.g., Wang et al. 1998, Bar-Matthews et al. 2003, 2010, Couchoud et al. 2009). Similar to the study of marine cores, an extensive arsenal of analytical methods have been applied to the study of lake cores, which serve as long, continuous archives of terrestrial climate change at annual to decadal scale for individual basins or watersheds. Existing lake cores in close proximity to paleoanthropological sites are typically restricted to the Holocene (e.g., Johnson & Odada 1996) but other cores in the Levant and Africa range from over 100 ka to 1 Ma (Koeberl et al. 2007, Scholz et al. 2007, Stein et al. 2011). Additional scientific drilling initiatives are exploring thick lacustrine deposits directly associated with Plio-Pleistocene paleoanthropological sites (Cohen et al. 2009)

Astronomical Controls on Long-Term Climate Change

The pattern of incident solar radiation (insolation) received on the planet at a given place and time is an important factor in understanding both directional trends and variability observed in many paleoclimatic records, particularly those related to Quaternary ice ages (Hays et al. 1976, Laskar et al. 2004). Changes in insolation are, in turn, driven by Earth's natural orbital oscillations, termed Milankovitch cycles. The three elements of Milankovitch cycles are eccentricity, obliquity, and precession (Figure 3). Eccentricity describes the degree of variation of the Earth's orbit around the Sun from circular to more elliptical. Eccentricity has two main periodicities, one cycle with an average of ~100,000 years and a longer cycle with a periodicity of ~413,000 years. Obliquity describes the tilt of the Earth's axis in relation to its orbital plane, which ranges from 22.1–24.5 degrees with a periodicity of ~41,000 years. Precession describes the motion of the Earth's axis of rotation, which does not point towards a fixed direction in the sky through time. Instead, the axis of rotation describes a clockwise circle in space, like the spinning of a wobbling top, with a periodicity of 19,000–23,000 years (Animation 1).

Animation 1: Earth’s orbital precession.

Which of these is the best description of a paleoclimate proxy? group of answer choices

Figure 3: Variations and schematic diagrams of Milankovitch cycles.

a, Precession and precessional index with a periodicity of ~23,000 years, with the amplitude of the cycles modulated at eccentricity periods of 100,000 years and 413,000 years (“variability packets”). b, The tilt of the Earth’s axis with a periodicity of 41,000 years. c, The eccentricity of the Earth’s orbit with periodicities of 100,000 and 413,000 years. d, Present position of the Earth in its orbit at different times of the year. e, Position of the Earth in its orbit at different times of year ~11,000 years in the future.

Solar radiation received at low-latitude is principally affected by variations in the cumulative effect of eccentricity and precession (eccentricity modulated precession), whereas higher latitudes are mainly affected by changes in obliquity. Since the Earth is tilted in its orbit, not all the Earth receives the same amount of energy, more energy being received at the equator than at the poles. Solar energy entering at a shallower angle at higher latitudes must travel further through the Earth's atmosphere compared to equatorial regions, reflecting some energy back to space. The same amount of solar energy also is spread over a larger area at higher latitudes. Increased tilt acts to amplify seasonal difference, while decreased tilt diminishes it. In its annual orbit, the Earth is currently closest to the sun (Perihelion) in early January, when the northern hemisphere is tilted away from the sun, and tilted towards the sun when the Earth is furthest from the sun (Aphelion) in early July (Figure 3d). Thus, seasonality is currently reduced in the northern hemisphere (but increased in the southern hemisphere) with the effect that northern hemisphere winters are not as cold as they could be, and summers are not as warm as they could be, a pattern that will be reversed in about 11,000 years (Figure 3e). Although the interactions between orbital parameters are major external drivers of paleoclimatic changes, the internal dynamics of the climate system also exert important controls on temporal and spatial patterns of environmental change. Furthermore, both external and internal forcing mechanisms can involve a complex series of feedbacks, and responses that may be linear or nonlinear, synchronous or delayed, or have a critical threshold ("tipping") point.

Paleoclimate and Hominin Evolution

One of the earliest examples that proposed a connection between climate-driven environmental change and hominin evolution was the "Savanna Hypothesis", which posited that the human lineage followed a simple trajectory from apelike to humanlike promoted by the challenges of an open savanna (Darwin 1871, Smith 1924, Bartholomew & Birdsell 1953). While we now know that there is no single "magic bullet" that is responsible for the multitude of anatomical and behavioral changes documented in the hominin record, the concept that certain changes in the human lineage may have evolved in open habitat settings has persisted. With the establishment of the marine paleoclimatic framework, researchers began to evaluate hominin evolutionary processes and events in the context of global climatic oscillations, particularly the onset of Northern Hemispheric Glaciation (NHG) ~2.7 Ma. The "Turnover Pulse Hypothesis" championed by paleontologist Elisabeth Vrba (Vrba 1988, 1995) proposed that a synchronous change in hominins, such as the origins of the genus Homo, and other African mammalian lineages, particularly speciation and extinction events in bovids, was caused by a shift from warm, moist conditions to cooler, drier, and more open habitats associated with a sharp transition in the marine oxygen isotope record associated with the onset of NHG (Figure 4). Other studies have since indicated that the record at specific East African hominin sites show either no faunal turnover at this time (e.g., Kingston et al. 1994) or that there were multiple pulses or prolonged periods of turnover set with a more gradual shift from forested to more open habitats (Behrensmeyer et al. 1997).

Which of these is the best description of a paleoclimate proxy? group of answer choices

Figure 4: Range chart of first and last appearance datums (FAD/LAD) of African fossil bovids spanning the last 7 Myr.

The dashed line represents a theoretical "null hypothesis" assuming a uniform rate of faunal turnover (speciation) set at 32% per million years. Notable faunal “turnover pulses”, clusters of origination and extinction events, which occurred near 2.8 Ma and 1.8 Ma were also associated with appearances of arid-adapted fauna.

A seminal study of terrigenous dust in marine cores off the coast of Africa by paleoceanographer Peter deMenocal suggested that subtropical African climate oscillated between markedly wetter and drier conditions, paced by Earth's orbital variations, with step-like increases in climate variability and aridity near 2.8, 1.7 and 1.0 Ma (deMenocal 1995, 2004). These steps were coincident with changes in the dominant orbital cycles from precession to obliquity to eccentricity, and with the onset and intensification of high-latitude glacial cycles, respectively. Compared to the African fossil and geological record, these time periods also coincided with proposed diversification points in the hominin lineage (2.9–2.4 Ma), paleoenvironmental evidence for drier habitats and the expansion of Homo out of Africa (1.8–1.6 Ma), and the extinction of the Paranthropus lineage, the broadened range of Homo erectus, and the establishment of more modern savanna ecosystems (1.2–0.8 Ma) (Figure 5). In addition to unidirectional shifts, deMenocal also highlighted the importance of "variability packets" of high- and low-amplitude paleoclimatic variability lasting 10,000 to 100,000 years in duration, paced by the orbital eccentricity modulation of precession (Figure 3a). These alternating periods of relative paleoclimatic stability (low eccentricity) and instability (high eccentricity) as a mechanism for introducing genetic variance to natural selection are a key component of the "Variability Selection Hypothesis" (Potts 1998), which proposes that the wide variability in adaptive settings over time ultimately favored complex adaptations that were responsive to novel conditions (i.e., the evolution of adaptability).

Which of these is the best description of a paleoclimate proxy? group of answer choices

Figure 5: Summary diagram of important paleoclimatic and hominin evolution events during the Plio-Pleistocene.

Gray bands indicate periods when African climate became progressively more arid after step-like shifts near 2.8 (±0.2) Ma and subsequently after 1.7 (±0.1) Ma 1.0 (±0.2) Ma coincident with the onset and intensification of high-latitude glacial cycles. From left to right: Percent of terrigenous dust in ODP site 721/722 with corresponding shifts in the dominant periodicity of the dust flux linked to precessional variability (23–19 kyr) and characteristic glacial cycles (41 kyr and 100 kyr). Approximate first and last appearance datums and possible relationships among hominin taxa. Soil carbonate carbon isotopic data from East African hominin localities documenting a progressive shift from woodland to grassland vegetation. Composite benthic foraminfera oxygen isotope record illustrating the evolution of high-latitude glacial cycles and dominant periodicity of glacial variability.

Studies of East African lake records by geologist Martin Trauth and colleagues have also focused on critical intervals near 2.6, 1.8 and 1.0 Ma and documented the presence of large, but fluctuating lakes, indicating consistency in wetter and more seasonal conditions every 800,000 years (Trauth et al. 2005, 2007). African monsoon intensity correlates with precession-paced insolation, and increased polar ice-volume acts to accentuate the pole-Equator thermal gradient, which leads to a north-south compression of the Intertropical Convergence Zone (ITCZ), the major control of monsoonal precipitation patterns in Africa. Associated with major glacial events near 2.6, 1.8 and 1.0 Ma, Trauth and colleagues propose that global climate changes led to increased seasonality and regional climate sensitivity to insolation, which resulted in packages of precessionally forced alterations between episodes of large lakes and extreme aridity, possibly as rapid as every ~10,000 years during eccentricity maxima (Trauth et al., 2003; Kingston et al., 2007). They propose that these occurred during periods of eccentricity maxima every 800,000 years since 2.7 Ma (similar to deMenocal's variability packets). While some East African lake records provide strong evidence for this pattern (e.g., Kingston et al. 2007), it may not be universal across space or time (Scholz et al. 2007). Ultimately, this hypothesis proposes that periods of dramatic climatic oscillations between 2.7–2.5 Ma, 1.9–1.7 Ma, and 1.1–0.9 Ma led to rapid expansion then subsequent contraction/fragmentation of hominin habitats at precessional timescales with associated dispersal events and vicariance in the hominin lineage (Figure 6).

Which of these is the best description of a paleoclimate proxy? group of answer choices

Figure 6: Summary diagram of global climate transition, East African lake occurrences and soil carbonate records, and hominin evolution.

East African lake occurrences are suggested to cluster during eccentricity maxima prior to 2.7 Ma (prior to NHG) and during periods of global climate transitions associated with eccentricity maxima after 2.7 Ma (post NHG). Note that lake phases do not occur during all eccentricity maxima and that some occur during eccentricity minima. Hominin FAD/LADs should be considered approximate.

Discussion and Challenges

It seems intuitive that large-scale shifts and short-term variability in paleoclimate altered local to regional hominin habitats and resource availability that ultimately led to selection pressures on our fossil ancestors. However, climate systems are markedly complex and dynamic, and may change drastically over relatively short distances. It is important to maintain a critical perspective on the types, quality, and scale of empirical paleoenvironmental data, particularly when the volume and temporal resolution of proxy data far exceeds that of the hominin fossil record itself (Kingston 2007, Behrensmeyer et al. 2007). For instance, error-bars on hominin FADs and LADs that indicate the probability of true origination or extinction events are rarely reported (e.g., Figures 5 & 6). When accounting for influences such as sample size and geochronological uncertainties, the potential mismatch between a taxon's actual origination and its documented FAD in the fossil record (or extinction and LAD) is likely on the order of tens to hundreds of thousands of years. All hypotheses that propose causal links between paleoclimatic change and hominin evolution must ultimately reconcile global patterns with local responses, and extend far beyond a general temporal correlation between environmental change and an evolutionary event. Criteria for testing hypotheses of environmental forcing include a highly resolved time scale for the various records to validate cause-before-effect order, a robust correspondence between multiple lines of proxy evidence that shows similar patterns or trajectories, the ability to rule out alternative (non-environmental) hypotheses, and ultimately, a causal mechanism. Nevertheless, once the assumptions and limitations of utilizing global paleoclimatic data are appreciated, the almost dizzying array of natural archives of the past provide paleoanthropologists with a highly-resolved contextual framework within which they can develop research questions and test hypotheses.