Modern calibration of <i>Poa flabellata</i> (tussac grass) as a new paleoclimate proxy in the South Atlantic

Abstract

Terrestrial paleoclimate records are rare in the South Atlantic, limiting opportunities to provide a prehistoric context for current global changes. The tussock grass, Poa flabellata, grows abundantly along the coasts of the Falkland Islands and other subantarctic islands. It forms extensive peat records, providing a promising opportunity to reconstruct high-resolution regional climate records. The isotopic composition of leaf and root tissues deposited in these peats has the potential to record variation in precipitation, temperature, and relative humidity over time, but these relationships are unknown for P. flabellata. Here, we measured the isotopic composition of P. flabellata and precipitation and explore relationships with seasonal temperature and humidity variations across four study locations in the Falkland Islands. We reveal that inter-seasonal differences in carbon and oxygen stable isotopes of leaf α-cellulose of living P. flabellata correlated with monthly mean temperature and relative humidity. The carbon isotope composition of leaf α-cellulose (δ13Cleaf) records the balance of CO2 supply through stomata and the demand by photosynthesis. The positive correlation between δ13Cleaf and temperature and negative correlation between δ13Cleaf and relative humidity suggest that photosynthetic demand for CO2 relative to stomatal supply is enhanced when conditions are warm and dry. Further, the positive correlation between δ13Cleaf and δ18Oleaf (r=0.88; p<0.001; n=24) indicates that stomatal closure during warm dry periods explains seasonal variation in δ13Cleaf. We observed significant differences between winter and summer seasons for both δ18Oleaf and δ13Cleaf and among study locations for δ18Oleaf but not δ13Cleaf. δ18O values of monthly composite precipitation were similar between seasons and among study locations, yet characteristic of the latitudinal origin of storm tracks and seasonal winds. The weak correlation between δ18O in monthly composite precipitation and δ18Oleaf further suggests that relative humidity is the main driver of the δ18Oleaf. The oxygen isotopes in root α-cellulose did not reflect, or only partially reflected (at one study location), the δ18O in precipitation. Overall, this study supports the use of peat records formed by P. flabellata to fill a significant gap in our knowledge of the long-term trends in Southern Hemisphere climate dynamics.