Atmospheric ¹⁴C Variability Recorded in Tree Rings from Peninsular India: Implications for Fossil Fuel CO₂ Emission and Atmospheric Transport

Abstract

Radiocarbon analysis in annual rings of a teak tree (Tectona grandis) is reported in comparison with previously published results. Samples (disks) were collected from Hoshangabad (22°30′N, 78°E), Madhya Pradesh, in central India. The previously published sample was collected from Thane (19°12′N, 73°E), Maharashtra, near the west coast of India (Chakraborty et al. 1994). Two short Δ14C time series were reconstructed with these tree samples to capture the bomb peak of atmospheric 14C and the spatial variability in this record. These time series represent the periods 1954–1977 and 1959–1980 for Hoshangabad and Thane, respectively. The 14C peaks in these places appear around 1964–1965. The Hoshangabad tree records a peak Δ14C value of 708 ± 8%, which conforms to the peak value of Northern Hemisphere Zone 3 as described in Hua and Barbetti (2004). But the peak Δ14C at Thane is somewhat less (630 ± 8%) probably due to the dilution by fossil fuel CO2 free of 14C emanating from the neighboring industrial areas. This depletion of peak values has been used to estimate the local emission of fossil fuel CO2, which is approximately 2.3% of the background atmospheric CO2 concentration.

Radiocarbon in the Northern Indian Ocean two decades after GEOSECS

“…This difference became insignificant after two decades, when the surface Arabian Sea recorded D 14 C values of 40 to 58‰ during 1994 and 1995, compared to 40 to 57‰ in the Andaman Sea and the southern Bay of Bengal during 1997 and 1999 ( Figure 5) [Bhushan et al, 2000;Dutta et al, 2007]. The reduction in the surface D 14 C values can be partly attributed to the lowering of the atmospheric D 14 C from $300‰ during 1978 [Nydal and Lövseth, 1996;Chakraborty et al, 2008] to $90‰ in late 1990s [Bhushan et al, 1997;Dutta et al, 2006], and transfer of 14 C-enriched waters from the surface to the deeper layers through vertical mixing. The later process appears to play a major role in the northern Indian Ocean as evidenced from the changes in the mean penetration depths of bomb-14 C.…”

“…The reduction in the surface D 14 C values can be partly attributed to the lowering of the atmospheric D 14 C from $300‰ during 1978 [Nydal and Lövseth, 1996;Chakraborty et al, 2008] to $90‰ in late 1990s [Bhushan et al, 1997;Dutta et al, 2006], and transfer of 14 C-enriched waters from the surface to the deeper layers through vertical mixing. The later process appears to play a major role in the northern Indian Ocean as evidenced from the changes in the mean penetration depths of bomb-14 C.…”

“…As far as tropical Indian Ocean is concerned, it is more appropriate to use the tropospheric D 14 C trend for the Northern Hemisphere Zone 3 as discussed in Hua and Barbetti [2004], where the peak D 14 C was $700‰ in 1965. Chakraborty et al [2008] reported peak tropospheric D 14 C of 708 AE 8‰ from tree rings of central India grown in 1965. In this study we have adopted a peak D 14 C of 700‰.…”

“…Here Q 14 is expressed in mol m C from 1954 and 1968 has been adopted from the reported measurements from the tropical regions [Nydal and Lövseth, 1996;Hua and Barbetti, 2004;Chakraborty et al, 2008], with peak D C mix is taken as 1400 [Stuiver, 1980]. From 1973 onward, a linear decrease of surface ocean D 14 C has been assumed, from a value of 140‰ in 1973 to 50‰ in 2000, to match with the observed values from the GEOSECS measurements during 1977-1978[Stuiver and Östlund, 1983 and during late 1990s [Dutta et al, 2006].…”

Decadal Variations in Oceanic Properties of the Arabian Sea Water Column since Geosecs

“…This is due to the steady decrease in atmospheric 14 C concentration since GEOSECS as the Δ 14 C of the atmosphere was much higher during GEOSECS than during this study. The average Δ 14 C of atmospheric CO 2 in the maritime air over the Arabian Sea was 120‰ in 1993 and 99‰ in 1998 (Bhushan et al 1997;Dutta et al 2006), compared to 313‰ in 1977 (during GEOSECS) as recorded in tree rings from peninsular India (Chakraborty et al 2008). However, the subsurface Δ 14 C values at most of the stations in the upper 1000 m were higher than the GEOSECS values due to increased penetration of bomb C values at this station since GEOSECS at depths between 1200-2200 m is most likely due to gradual sinking of more saline and dense Red Sea surface water (Bhushan et al 2003;Dutta and Bhushan 2012).…”