New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 62

future relevance. From a philosophical perspective, our understanding of geosphere-
biosphere interactions in the past shapes our basic curiosity of where humans come
from and our perception of human's role in the world.

Box 2.8
Proxies for Reconstructing Past Climates
Reconstructing past climates rests on our ability to indirectly infer temperature,
precipitation, atmospheric greenhouse gas concentrations, and other environmental
properties from sedimentary materials. The best-known proxy is

18 O of biogenic CaCO 3 (in
marine microfossils and animals), which has long been shown to reflect the combined effects
of local temperature and global ice volume on seawater

δ

18 O. More recently, oxygen isotope
analysis of biogenic hydroxyapatite in marine and terrestrial fossils has been utilized as a
proxy of seawater 

δ

18 O and of continental mean annual temperatures, respectively (e.g.,
Fricke and Wing, 2004; Buggisch et al., 2008; Trotter et al., 2008). During the past decade, a
variety of new proxies have been developed that have led to a major improvement in our
ability to reconstruct past climates (summarized in Understanding Earth's Deep Past [NRC,
2011a]).

δ

Despite the maturity of the stable isotope field, fundamentally new advances continue
to be made—for example, by assessing the distribution or “clumping” of rare isotopes in
minerals. Traditionally, the isotopic composition of a compound is determined by destroying
the original structure of that compound and measuring the relative isotopic abundances of the
bulk material. For example,

18 O of calcite document the 13 C/ 12 C and 18 O/ 16 O ratios
in the sample, retaining no record of how those isotopes were distributed. Recent advances
that allow access to this distribution have ushered in a new and rich source of information
contained in the stable isotopes. Most notably, Ghosh et al. (2006) showed that there is a
temperature-dependent thermodynamic preference for heavy isotopes in calcite to share a
bond—the lower the temperature, the stronger the preference for 13 C- 18 O bonds compared to
a completely random distribution. This discovery forms the basis of a completely new type of
calcite paleothermometer. In particular, a measurement of the abundance of the 13 C

13 C and

δ δ

18 O 16 O
variant of CO 2 evolved from calcite relative to the random distribution of isotopes, referred to
as Δ47, can provide formation temperatures to a precision of ±2 ° C. Importantly, unlike
classical

δ

18 O calcite thermometry, this “clumped isotope thermometer” is independent of
assumptions about the composition of water from which the calcite precipitated.
Recent work demonstrates that clumped isotopes accurately record
paleotemperatures in a wide variety of marine biogenic carbonates (Came et al., 2007; Tripati
et al., 2010), cave and soil carbonates (Affek et al, 2008; Passey et al., 2010), and
carbonate-fluorapatite in vertebrate bones (Eagle et al., 2010). Ongoing work (e.g., Passey
et al., 2011) reveals an apparent sensitivity of clumped isotopes in low-temperature
precipitates to diagenesis requiring further calibration and assessment studies.

δ