Chapter Five Radiocarbon Dating of Deep-Sea Sediments

Abstract

This chapter discusses the radiocarbon dating of deep-sea sediments. Deep-sea sediments typically provide continuous deposition with few hiatuses, despite large shifts in climate and depositional regime. In addition, marine sediment archives offer opportunities for application of multiple proxies representing different climatic and oceanographic systems. Radiocarbon dating has served as a powerful tool for accurate dating of marine records, and can be used as a geochemical tracer of climate and carbon cycle changes. The chapter focuses on the geochronological use of natural 14C activities. The two main fields of radiocarbon dating originate: (1) photosynthesis fixes CO2, and hence 14CO2, into plant organic matter, allowing dating of organic carbon samples, and (2) CO2 dissolves in sea water to form dissolved inorganic carbon (DIC), which can be dated directly or following incorporation into marine carbonates. 14C dating is based on the radioactive decay law. The 14C activity or ratio of 14C to total C in a sample compared to that in a standard is either measured by detection of its radioactive decay (radiometric techniques), or by counting carbon ions using accelerator mass spectrometry (AMS). Atmospheric 14C enters the ocean as dissolved CO2, CO2(aq), which reacts with water to form carbonic acid, and then dissociates into bicarbonate and carbonate ion forms. The base 14C activity of the biogenic carbonate pool of marine fauna is set by the mixed-layer carbonate species. In some circumstances, the 14C dating of recognizable plant or carbonate macrofossils is not possible owing to low productivity or lack of preservation, and researchers turn to the pool of organic carbon as material for 14C dating.