Stable Isotope Geochemistry III: Low Temp Applications - Isotopes & Diet, Study notes of Geochemistry

Download Stable Isotope Geochemistry III: Low Temp Applications - Isotopes & Diet and more Study notes Geochemistry in PDF only on Docsity! Geol. 656 Isotope Geochemistry Chapter 10 276 8/22/12 Stable Isotope Geochemistry III: Low Temperature Applications STABLE ISOTOPES IN PALEONTOLOGY, ARCHEOLOGY, AND THE ENVIRONMENT Introduction The isotopic composition of a given element in living tissue depends on: (1) the source of that element (e.g., atmospheric CO2 versus dissolved CO2; seawater O2 vs. meteoric water O2), (2) the processes in- volved in initially fixing the element in organic matter (e.g., C3 vs. C4 photosynthesis), (3) subsequent fractionations as the organic matter passes up the food web. Besides these factors, the isotopic compo- sition of fossil material will depend on any isotopic changes associated with diagenesis, including mi- crobial decomposition. In this lecture, we will see how this may be inverted to provide insights into the food sources of fossil organisms, including man. This, in turn, provides evidence about the environ- ment in which these organisms lived. Isotopes and Diet: You are what you eat In Chapter 8 we saw that isotope ratios of carbon and nitrogen are fractionated during primary pro- duction of organic matter. Terrestrial C3 plants have δ13C values between -23 and -34‰, with an av- erage of about -27‰. The C4 pathway involves a much smaller fractionation, so that C4 plants have δ13C between -9 and -17‰, with an average of about -13‰. Marine plants, which are all C3, can utilize dissolved bicarbonate as well as dissolved CO2. Seawater bicarbonate is about 8.5‰ heavier than at- mospheric CO2; as a result, marine plants average about 7.5‰ heavier than terrestrial C3 plants. In con- trast to the relatively (but not perfectly) uniform isotopic composition of atmospheric CO2, the carbon isotopic composition of seawater carbonate varies due to biological processes. Because the source of the carbon they fix is more variable, the isotopic composition of marine plants is also more variable. Finally, marine cyanobacteria (blue-green algae) tend to fractionate carbon isotopes less during photo- synthesis than do true algae, so they tend to average 2 to 3 ‰ higher in δ13C. Nitrogen isotopes are, as we saw, also frac- tionated during primary uptake. Based on their source of nitrogen, plants may also be di- vided into two types: those that can utilized N2 directly, and those utilize only “fixed” nitrogen as ammonia and nitrate. The former include the legumes (e.g., beans, peas, etc.) and marine cyanobacteria. The legumes, which are exclu- sively C3 plants, utilize both N2 (through sym- biotic nitrogen-fixing bacteria in their roots) and fixed nitrogen, and have an average δ15N of +1‰, whereas modern nonleguminous plants average about +3‰. However, it seems likely that prehistoric nonleguminous plants were more positive, averaging perhaps +9‰, be- cause the isotopic composition of present soil nitrogen has been affected by the use of chemi- cal fertilizers. For both groups, there was Figure 10.01. Relationship between δ13C and δ15N among the principal classes of autotrophs. Docsity.com Geol. 656 Isotope Geochemistry Chapter 10 277 8/22/12 probably a range in δ15N of ±4 or 5‰, because the isotopic composition of soil nitrogen varies and there is some fractionation involved in uptake. Ma- rine plants have δ15N of +7 ±5‰, whereas marine cyanobacteria have δ15N of –1±3‰. Figure 10.01 summarizes the isotopic composition of nitrogen and carbon in the various classes of photosynthetic organisms (autotrophs). DeNiro and Epstein (1978) studied the relation- ship between the carbon isotopic composition of animals and their diet. (Most of the animals in this study were perhaps of little direct paleontological interest, being small and soft-bodied. DeNiro and Epstein studied small animals for a practical rea- son: they are easier to analyze than a large animal such as a horse.) Figure 10.02 shows that there is lit- tle further fractionation of carbon by animals and thus the carbon isotopic composition of animal tis- sue closely reflects that of their diet. DeNiro and Epstein (1978) estimteed that carbon in animal tis- sue is on average about 1‰ heavier than their diet. Mice, although not analyzed whole and not shown in Figure 10.02, were also included in the study. Various tissues from mice had δ13C within ±2‰ of their diet, so the relationships in Figure 10.02 ex- tend to vertebrates as well. DeNiro and Epstein found that the same species has a different isotopic composition when fed a different diet. Conversely, different species had similar isotopic compositions when fed the same diet. Thus diet seems to be the primary control on the isotopic composition of animals. Subsequent work by McCutchan et al. (2003) found an average shift in δ13C of animals relative to their diet of +0.5±0.13. The small fractionation between animal tissue and diet is a result of the slightly weaker bond formed by 12C compared to 13C. The weaker bonds are more readily broken during respiration, and, not surprisingly, the CO2 respired by most animals investigated was slightly lighter than their diet. Thus only a small fractionation in carbon isotopes occurs as organic carbon passes up the food chain, about +1‰ at each step in the chain. Terrestrial food chains are usually not more than 3 trophic lev- els long, implying a maximum further fractionation of +3‰. Marine food chains can have up to 7 tro- phic levels, implying a maximum carbon isotope difference between primary producers and top predators of 7‰. These differences are smaller Figure 10.02. Relationship between δ13C in an- imals and that of their diets. When a species was given more than one diet, that diet is shown in parentheses. After DeNiro and Epstein (1978). Figure 10.03. Relationship between δ15N in an- imals and that of their diet. Animals typically have 3-4‰ heavier nitrogen that their diet. Dashed line is the isotopic composition plus 3.5‰. When a species was given more than one diet, that diet is shown in parentheses. After DeNiro and Epstein (1981). Docsity.com Geol. 656 Isotope Geochemistry Chapter 10 Spring 2011 280 8/22/12 to the evolution of grasslands? If that is the case, it is difficult to understand the change in dental mor- phology. Alternatively, grasslands may have appeared in mid-Miocene and only subsequently become dominated by C4 grasses. If the latter interpretation is correct, it raises the question of what evolutionary pressure caused the change from C3 to C4 photosynthesis in tropical and temperate grasses. An important observation in that respect is that C4 grasslands appear to have become important in both North America and Asia at about the same time (7 Ma). Indeed, the first evidence for a shift from C3 dominant to C4 dominant eco- systems came from an observed change in the δ13C of soil carbonate in Pakistan (Quade et al., 1989). Quade et al. (1989) first interpreted this as a response to the uplift of the Tibetan Plateau and the devel- opment of the Monsoon. However, other evidence, including oxygen isotope data from Pakistani soil carbonates, suggests the Monsoons developed about a million years earlier (about 8 Ma). The synchro- nicity of the C4 grass becoming dominant in the grasslands in Asia and North America (Figure 10.07) suggests a global cause, while the Monsoons are a re- gional phenomenon. Furthermore, subsequent 14C stud- ies of the teeth of horses and other mammals revealed that, at least in Tibet, uplift of the plateau was associated with a shift from C4 to C3 vegetation (Wang et al., 2006). Though there has been some speculation that the C4 photosynthetic pathway may have evolved as early as the Cretaceous, the oldest direct fossil evidence for C4 plants (plants with enlarged bundle-sheath cells) is late Miocene; i.e., the same age as the observed δ13C increase. Thus the isotopic shift may date the evolution of C4 photosynthesis. C4 photosynthesis involves only relatively minor modifi- cation of plant enzymes and structures and it occurs in di- verse, distantly related families. It may, therefore, have evolved independently in many families (Ehlerginer, et al. 1991). This also suggests some global environmental change that favored C4 photosynthesis. Several groups have now suggested that the appear- ance of C4 grasses reflects a drop in the concentration of Figure 10.07. δ13C in carbonates from paleosols of the Potwar Plateauin Paki- stan. The change in δ13C may reflect the evolution of C4 plants. From Quade et al. (1989). Figure 10.06. δ13C and crown height in North American fossil horses as a function of age. After Wang et al. (1994). Docsity.com Geol. 656 Isotope Geochemistry Chapter 10 Spring 2011 281 8/22/12 atmospheric CO2 in the Miocene. In the C3 photosyn- thetic pathway, Rubisco can catalyze not only the fixation of carbon in phosphoglycerate, but also the reverse reaction where CO2 is released, a process called photorespiration. When concentrations of CO2 are high, the forward reaction is favored and the C3 pathway is more efficient overall than the C4 path- way. At low CO2 concentrations, however, the C4 pathway, in which CO2 is first transported into bun- dle-sheath cells, is more efficient, as the concentration in bundle-sheath cells is maintained at around 1000 ppm (Figure 10.08). Thus under present conditions, C4 plants have a competitive advantage. At higher CO2 conditions, C3 plants are more efficient. There is some evidence that Eocene CO2 concentrations were much higher than present (perhaps 800 ppm as op- posed to 250 ppm pre-Industrial Revolution), and that concentrations dropped dramatically during the Miocene (however, boron isotope evidence, which we will discuss in a subsequent lecture, suggests the drop in CO2 concentrations occurred in the Eocene). Such a drop would give C4 plants a competitive ad- vantage. This would be particularly true in the warm climates where C4 plants dominate because the rate of photorespiration is temperature dependent. An interesting epilogue to these studies is that of MacFadden et al., (1999) who analyzed both carbon isotopes and abrasion patterns of 6 species of early Pliocene (~ 5 Ma) horses from Florida. All six spe- cies bore the high-crowned dental hallmark of grazers. Some species grazed solely on C4 grasses, but others, notably including D. mexicanus, a close relative of the modern horse, browsed C3 shrubs and trees. The high crowned dental morphology had been inherited from their ancestors and when the spe- cies switched back to eating browse, its teeth did not change. This is an example that evolution is often “irreversible”. Were Dinosaurs Warm-Blooded? Dinosaurs were long-thought to be cold-blooded like their reptile relatives. Nearly half a century ago, however, some paleontologists began to suspect that this might not be the case. The realization that birds, which are warm-blooded, are descendent from dinosaurs and the discovery that at least one group of dinosaurs had feathers, has furthered these suspicions. Casting the issue in terms of warm- vs. cold-blooded, however, may be a bit misleading. Some dinosaurs were so large that the real issue would have been keeping body temperature down, not up. A better question is whether dinosaurs were homeothermic, that is whether they could maintain a more or less constant and body tempera- ture. One approach to the problem has been to compare oxygen isotope ratios of biogenic phosphate (in bones, teeth, etc.) from dinosaurs with that of reptiles known to be cold-blooded at variety of latitudes. To understand this approach, we need to recall two observations from Chapter 8: isotopic fractionation decreases with increasing temperature (including the fractionation between apatite and body water) and δ18O in precipitation decreases with mean annual air temperature and latitude (Figure 8.10). Iso- topic fractionation will be small when internal body temperature is high, in which case δ18O in phos- phate will be similar to that in precipitation. This would be the case in heterothermic (cold-blooded) animals living in warm climates at low latitude and in homeothermic animals living at any latitude. In Figure 10.08. Rate of photosynthesis as a func- tion of intercellular CO2 concentrations in C4 and C3 plants. At concentrations of atmos- pheric CO2 that prevailed before the Industrial Revolution, C4 plants would have had a com- petitive advantage. At concentrations above the present level, C3 plants are more efficient. From Ehleringer et al. (1991). Docsity.com