12 - Isotope and Carbon Cycle Inferences

Summary

Abstract

Various applications of carbon isotope (¹³C and ¹⁴C) records are described. The main data sources are dendrochronologically dated tree rings, ice cores, and ocean sediments (including corals). The representativeness and characteristics of these records are discussed. The history of atmospheric ¹⁴C changes is determined by changes in oceanic upwelling rate and by solar and geomagnetic influences on upper atmosphere production rate. Separating these causal factors from the record is difficult, but analyses suggest interesting cyclic changes in North Atlantic deep water formation rates (periodicity around 500 years) and solar output (periodicity around 200 years). Isotopic data have provided valuable oceanic information regarding the current atmosphere-to-ocean flux of CO₂, deep water residence times, current upwelling rates, and glacial/interglacial changes in upwelling rate. This work is discussed and evaluated. Finally, the problems involved in interpreting radiocarbon dates in terms of calibrated (i.e., estimated calendar) dates are illustrated using the dating of the Mazama (U.S. Pacific Northwest) eruption as an example.

Introduction

Natural carbon contains the three carbon isotopes ¹²C, ¹³C, and ¹⁴C. Of these isotopes, ¹²C is by far the most abundant at 98.9% of total carbon. Thus, the carbon cycle in nature is essentially a ¹²C cycle, with ¹³C (1.1%) and ¹⁴C (10^–10%) contributing only minor amounts. Nevertheless, ¹³C and ¹⁴C play a major role as tracers through which information on the physical and chemical properties of the carbon cycle can be obtained.