3.7 Conclusions
3.7.1 Future Directions
This work is currently being prepared for publication in Climate of the Past in collaboration with Allegra LeGrande and Jesse Nusbaumer at NASA GISS, Nerilie Abram at the Research School of Earth Sciences, ANU, and Sophie Lewis at the Fenner School of the Environment, ANU. In preparation for publication, there are additional changes and analyzes to be made. First, to consolidate the many data sets that have been utilized in this study, I will replace the IsoGSM amount effect analysis with data from the isotope-enabled, MERRA2-forced run of GISS ModelE2- R NINT. This new model will allow for a more direct comparison of the modern precipitation source distributions of each site with their local amount effect. Additionally, GISS ModelE2-R is currently being run at 1 ky time slices since the Last Glacial Maximum with ice volume and topography configured for those slices. I will be comparing the PSDs and δ18Op patterns from LGM (~19 ka), Heinrich
114
Stadial 1 (~17 ka), and Younger Dryas (~12 ka) time slices. In particular, I am interested if small changes in topography between HS1 and the YD could be responsible for the hydroclimate sensitivity or lack thereof, at sites very near to the exposed Sunda Shelf. Likewise, I would like to compare LGM and YD PSDs to examine how the prevailing circulations in the IPWP were different. There is much debate over whether the LGM was as wet as the Holocene or if glacial aridity
prevailed in the IPWP (Konecky et al., 2016).
This study was largely qualitative; HYSPLIT and PSDs allow us to examine moisture origins and relative global abundance but are not conducive to quantifying the fraction and isotopic values of waters arriving at a site from a particular source. The work presented here suggests that considering moisture source and fractionation along the moisture transport path are critical to
interpreting isotopic signatures to a paleoproxy site, particularly at sites with complicated seasonality. In our future work, we will use tagged water regions based on the HYSPLIT and PSD results to quantify the isotopic signature of the waters arriving from the identified source regions in both modern and paleo-runs. This will help address assumptions of stationarity in the source regions and the amount effect over time.
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4
Aragonite-calcite speleothem
petrography and geochemistry
4.1
Introduction
Speleothems are secondary cave deposits that most commonly precipitate as either calcite or the metastable CaCO3 polymorph, aragonite (Hill and Forti, 1997). Most paleoclimatic reconstructions use calcite stalagmites because aragonite is thermodynamically unstable and therefore subject to variable post-depositional alteration to calcite (Finch et al., 2001; Frisia et al., 2002). U/Th dating and interpretation of isotopes and trace elements becomes ambiguous in secondary calcite because the transformation takes place in an open system (Bajo et al., 2016; Frisia et al., 2002; Lachniet et al., 2012). However, if no diagenetic alteration has taken place, aragonite speleothems can be robust archives for paleoclimatic studies, in part due to their high uranium concentrations, providing the
opportunity for excellent age models with small uncertainties (Zhang et al., 2013). As stable aragonite can persist indefinitely, aragonitic speleothems have already been successfully employed in paleo-reconstructions (Cosford et al., 2008; Li et al., 2011a).
Aragonite stalagmites tend to form during slow drip rates and when the aqueous Mg/Ca value is greater than unity (Frisia et al., 2002). As such, aragonite usually precipitates from Mg-rich dolomitic host rocks rather than limestone (Railsback et