Spatial and temporal changes in bomb radiocarbon in the northern Indian Ocean

Highlights

• Comparison of temporal changes in surface Δ¹⁴C value of the northern Indian Ocean.

• Northern Andaman shows enriched Δ¹⁴C value compared to the atmosphere.

• Δ¹⁴C decline rate in the Southern Bay of Bengal is lower than the Northern Andaman.

• Lakshadweep and Southern Bay of Bengal show comparable surface Δ¹⁴C values.

Abstract

For improved understanding of ocean circulation in the northern Indian Ocean region, long term continuous record of radiocarbon measurement is required. Limited radiocarbon records from the region demands investigations of natural archives. Coral core records along with some literature data were analysed to study the temporal changes in ¹⁴C values over the northern Indian Ocean. The major fraction of the bomb radiocarbon appears to have transferred in to the ocean, as recent records from the surface seawater Δ¹⁴C values show comparable or even higher than the atmospheric Δ¹⁴C values. The northern Andaman region showed higher Δ¹⁴C decline rate between 1978 and 2014 compared to the southern Bay of Bengal and the Lakshadweep region. The comparable southern Bay of Bengal and the Lakshadweep Δ¹⁴C values could be due to transfer of Arabian Sea waters to the southern Bay of Bengal. The southern Andaman region shows lower Δ¹⁴C values compared to the northern Andaman region, suggesting the influence of ¹⁴C depleted waters in the region.

Introduction

Radiocarbon is a radioactive isotope of carbon produced in the upper atmosphere. After its production, it rapidly converts to ¹⁴CO₂ and then mixes with seawater thereby bringing radiocarbon to the ocean. Several nuclear tests carried out during 1950s led to the introduction of large amount of artificially produced radiocarbon termed as bomb-radiocarbon into the atmosphere (Hua et al. 2000, 2013). These nuclear tests led to drastic increase in the atmospheric radiocarbon concentration in the northern hemisphere, which almost doubled by 1963 and subsequently started decreasing (Levin et al., 1985; Hua et al., 2013; Graven 2015). This bomb radiocarbon is sequestered in to the ocean by air-sea CO₂ exchange processes. Bomb radiocarbon showed spike in the surface seawater similar to the atmosphere, however the oceanic spike was smaller in magnitude and had a lag of several years (Druffel and Linick 1978; Guilderson et al., 1998; Andrews et al., 2016). This lag and dampening of the radiocarbon signal in the ocean is because of relatively slow mixing and large volume of the ocean reservoir. The radiocarbon spike is followed by a declining trend in the radiocarbon concentration in surface seawater (Grumet et al., 2004; Mitsuguchi et al., 2007; Guilderson et al., 2009; Andrews et al., 2016). These temporal changes in surface seawater ¹⁴C help us to understand the processes like air-sea CO₂ exchange (Druffel and Suess 1983; Cember 1989), upwelling (Grumet et al., 2004) and ocean circulation (Druffel and Griffin 1993; Guilderson et al., 2009). Radiocarbon changes both in the atmosphere and the ocean are useful to understand carbon cycles and the oceanic process regulating the distribution of both natural and bomb-radiocarbon.

The Bay of Bengal is a very dynamic region in the eastern part of the northern Indian Ocean, which receives large freshwater fluxes (Sengupta et al., 2006) and experiences reversal in surface circulations (Shankar et al., 2002). The dynamic nature of this north-eastern Indian Ocean region makes it interesting for studying the radiocarbon variability over the region due to its characteristic oceanic processes. Few investigations in the Bay of Bengal waters have been conducted based on radiocarbon concentrations to understand the spatial and temporal changes in the basin (Dutta et al., 2010; Dutta and Bhushan 2012). Some of the earliest radiocarbon measurements were carried out during GEOSECS expedition in the Indian Ocean during 1977–78 (Stuiver and Östlund, 1983). Later, radiocarbon measurements in the Indian Ocean were conducted during WOCE expedition between 1994–95 (Key and Quay 2002) and during oceanographic expeditions by Physical Research Laboratory (PRL) between 1994–99 (Somayajulu et al., 1999; Bhushan et al., 2003; Dutta et al., 2007; Dutta and Bhushan 2012). Dutta et al. (2010) reported ¹⁴C values from surface seawater of the Bay of Bengal during 2006 and compared them with previous expedition results. These observations provided a glimpse of temporal and spatial changes in the ¹⁴C values of surface seawater in the Bay of Bengal. A few studies on pre-bomb radiocarbon values were also made from the Bay of Bengal region (Dutta et al., 2001; Southon et al., 2002; Raj et al., 2020). Both pre-bomb and post-bomb radiocarbon show significant variability across the basin. Spatial and temporal variability in surface water ¹⁴C values can be studied to understand surface ocean processes in the region. Long term continuous temporal records of radiocarbon variability in the surface waters of the region are needed for such investigations.

To decipher the decadal changes of ¹⁴C in the surface water of the Bay of Bengal, coral cores from the northern Indian Ocean region were analysed to obtain a continuous ¹⁴C record. These records were compared with the available radiocarbon measurements from the region to understand the processes governing ¹⁴C changes of surface waters in the region.