High-resolution <sup>14</sup>C bomb peak dating and climate response analyses of subseasonal stable isotope signals in wood of the African baobab – a case study from Oman

Abstract

Abstract. The African baobab, Adansonia digitata L., has great paleoclimatological potential because of its wide distributional range and millennial length life span. However, dendroclimatological approaches are hampered by dating uncertainties due to its unique, parenchyma-dominated stem anatomy. Here, securely dated time series of annual wood increment growth and intra-ring stable isotopes of carbon and oxygen of cellulose for a baobab tree from Oman covering 1941 to 2005 were established and tested for relationships to hydroclimate variability. Precise dating with the atomic bomb peak (ABP) using highly resolved 14C measurements confirmed the annual character of the baobab's growth rings. F14C values of tree-ring cellulose were found up to 8.8 % lower than in the corresponding atmospheric CO2 for the period around the ABP, which in conjunction with a considerable autocorrelation of the δ13C series points to the incorporation of previous year's carbon contributing to the average age of intra-ring wood samples. F14C of terminal parenchyma bands, marking the tree-ring boundaries, were found to be considerably younger than their corresponding tree ring, indicating that parenchyma tissue is alive for many years, probably undergoing cell division and structural reorganization and contributing to secondary growth. In contrast to the δ13C time series, no significant autocorrelation was found in the δ18O series of tree-ring cellulose despite the enormous water storage potential of this stem-succulent tree species. Year-to-year variability in tree-ring width and stable isotope ratios revealed radial stem growth and the geochemistry of wood cellulose are influenced by fluctuations in the hydroclimate. In particular, δ18O was found to be a good climate proxy, followed by tree-ring width and δ13C. Tree-ring width and intra-ring δ18Omin correlated well with each other and with precipitation amount for the period from pre-monsoon May to the end of the monsoon season in September/October. Intra-annual stable isotope courses were found to be rather similar for both δ13C and δ18O. Years with particularly low monsoon rain were reflected by increased stable isotope values in the mid-section of intra-annual courses. Distinct patterns with low subseasonal isotope values seem indicative for years with heavy rainfall events from pre-monsoonal cyclones. Rain events from post-monsoonal cyclones may also be recorded; however, only 2 years of observation prevented a more conclusive evaluation.

What we talk about when we talk about seasonality – A transdisciplinary review

“…Similarly, the intra-seasonal variation in 18 O of cellulose in tree rings from the Quinghai-Tibetan Plateau reflects variation in the 18 O of precipitation associated with incursion of the Indian summer monsoon [630], the authors suggest that the minimum intra-seasonal tree ring 18 O value might be used to infer and reconstruct the timing of the local monsoon onset. Distinct subseasonal isotope patterns seem indicative for years with heavy rainfall events from pre-monsoonal cyclones in Oman [531], so that cyclonic activity can be potentially tracked back for hundreds of years [45].…”

A 900-Year Isotopic Proxy Rainfall Record from Northeastern Botswana

“…Baobab trees (Adansonia spp.) were used as rainfall proxy before by Woodborne et al [20,64,65], Slotta et al [40,85] and Razanatsoa [46], who obtained rainfall reconstructions for southwestern Madagascar spanning 775 years for Adansonia grandidieri and 765 years for A. za.…”

“…Solar activity, which shows a well-defined periodicity of grand minima/maxima cycles of 350-400 years played a role in shaping past climate variations before the Industrial Revolution [85,86]. Thus, events that marked the past climate of the last millennium are the Dalton Minimum (ca.…”