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

• Main Authors: D. V. Groff, D. G. Williams, J. L. Gill
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-09-01
• Series: Biogeosciences
• Online Access: https://bg.copernicus.org/articles/17/4545/2020/bg-17-4545-2020.pdf
• ISSN: 1726-4170; 1726-4189

<p>Terrestrial paleoclimate records are rare in the South Atlantic, limiting opportunities to provide a prehistoric context for current global changes. The tussock grass, <i>Poa flabellata</i>, 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 <i>P. flabellata</i>. Here, we measured the isotopic composition of <i>P. flabellata</i> 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 <span class="inline-formula"><i>α</i></span>-cellulose of living <i>P. flabellata</i> correlated with monthly mean temperature and relative humidity. The carbon isotope composition of leaf <span class="inline-formula"><i>α</i></span>-cellulose (<span class="inline-formula"><i>δ</i>¹³C</span><span class="inline-formula">_leaf</span>) records the balance of <span class="inline-formula">CO₂</span> supply through stomata and the demand by photosynthesis. The positive correlation between <span class="inline-formula"><i>δ</i>¹³C</span><span class="inline-formula">_leaf</span> and temperature and negative correlation between <span class="inline-formula"><i>δ</i>¹³</span>C<span class="inline-formula">_leaf</span> and relative humidity suggest that photosynthetic demand for <span class="inline-formula">CO₂</span> relative to stomatal supply is enhanced when conditions are warm and dry. Further, the positive correlation between <span class="inline-formula"><i>δ</i>¹³</span>C<span class="inline-formula">_leaf</span> and <span class="inline-formula"><i>δ</i>¹⁸O</span><span class="inline-formula">_leaf</span> (<span class="inline-formula"><i>r</i>=0.88</span>; <span class="inline-formula"><i>p</i>&lt;0.001</span>; <span class="inline-formula"><i>n</i>=24</span>) indicates that stomatal closure during warm dry periods explains seasonal variation in <span class="inline-formula"><i>δ</i>¹³C</span><span class="inline-formula">_leaf</span>. We observed significant differences between winter and summer seasons for both <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_leaf</span> and <span class="inline-formula"><i>δ</i>¹³C</span><span class="inline-formula">_leaf</span> and among study locations for <span class="inline-formula"><i>δ</i>¹⁸O</span><span class="inline-formula">_leaf</span> but not <span class="inline-formula"><i>δ</i>¹³C</span><span class="inline-formula">_leaf</span>. <span class="inline-formula"><i>δ</i>¹⁸</span>O 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 <span class="inline-formula"><i>δ</i>¹⁸O</span> in monthly composite precipitation and <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_leaf</span> further suggests that relative humidity is the main driver of the <span class="inline-formula"><i>δ</i>¹⁸O</span><span class="inline-formula">_leaf</span>. The oxygen isotopes in root <span class="inline-formula"><i>α</i></span>-cellulose did not reflect, or only partially reflected (at one study location), the <span class="inline-formula"><i>δ</i>¹⁸O</span> in precipitation. Overall, this study supports the use of peat records formed by <i>P. flabellata</i> to fill a significant gap in our knowledge of the long-term trends in Southern Hemisphere climate dynamics.</p>