The stable isotope biogeochemistry of ostrich eggshell and its application to Late Quaternary paleoenvironmental reconstructions in South Africa.

摘要

The stable carbon, nitrogen and oxygen isotope compositions of fossil ostrich eggshell (OES) were explored for paleodietary and paleoecologic information in Africa. Methodologies were determined for the isolation and isotopic analysis of the total organic fraction in OES. Modern OES samples were collected from a variety of environments, including those where the composition of the diet was known (i.e., zoos and commercial ostrich farms), and from a variety of ecosystems where the ostriches were free-ranging (South Africa), and analyzed for stable carbon, nitrogen and oxygen isotopes. A set of heating experiments was performed on modern OES to simulate geologic time and to evaluate the degree of isotopic alteration that occurs in OES during diagenesis.; The results of these experiments demonstrated that (1) the {dollar}deltasp{lcub}13{rcub}{dollar}C compositions of the total organic and inorganic fraction of OES are enriched relative to the diet by 2% and 14%, respectively, (2) the {dollar}deltasp{lcub}13{rcub}{dollar}C composition of some amino acids in OES are more fractionated relative to the diet than others, (3) the {dollar}deltasp{lcub}15{rcub}{dollar}N composition of the total organic fraction of OES is enriched by 3% relative to the diet and is inversely correlated with mean annual precipitation, (4) the {dollar}deltasp{lcub}18{rcub}{dollar}O composition of the inorganic fraction of OES reflects the isotope composition of the drinking water, and in certain settings appears to be inversely correlated with relative humidity, (5) the {dollar}deltasp{lcub}13{rcub}{dollar}C and {dollar}deltasp{lcub}15{rcub}{dollar}N composition of the total organic fraction in OES is stable during diagenesis, and (6) the {dollar}deltasp{lcub}13{rcub}{dollar}C compositions of glycine and alanine become enriched during diagenesis.; Based on these studies, Holocene paleoenvironmental information was derived from the stable isotope composition of fossil OES at Equus Cave, eastern Cape Province, South Africa. Results demonstrated that (1) ostriches subsisted mainly on C{dollar}sb3{dollar} plants throughout the Holocene, and (2) precipitation and temperatures were significantly decreased at 11 ka BP relative to the rest of the Holocene. The apparent decrease in precipitation and temperature at 11 ka BP can be explained by the effects of reduced insolation on the intensity of the South African monsoon.