This doctoral dissertation improvement project will study the relative importance of summer temperature and precipitation in controlling the carbon balance in peatlands in Alaska and southern Patagonia. Past shifts in the earth?s rotational axis have differentially affected temperatures in the northern and southern hemisphere. By examining past rates of carbon sequestration in cores from peatlands in both hemispheres the importance of past climate can be inferred.
High latitude regions are experiencing warming at rates faster than elsewhere on earth. Because warming affects both the production and the decay of organic matter in peatlands, it is unclear how warming will affect carbon storage in peatland soils, which account for about one-third of soil carbon globally. Understanding the fate of carbon stored in peatlands is key to assessing the global carbon budget.
We had three main goals: 1-estimate peatland carbon stocks and document Holocene peatland accumulation rates in Patagonia, 2-reconstruct peatland historical development and associated climatic conditions in Patagonia, and 3-compare Patagonian peatlands dynamics to that of northern peatlands. First, we analyzed peat-core data from 4 new study sites and from 19 publications. We found that Patagonian peatlands have been efficient land carbon sinks since their initiation, with a mean soil carbon density of 168 kg of carbon / m2. The total carbon pool for these ecosystems was estimated at 7.6 gigatons. We estimated long-term peat accumulation rates, peat addition rates, and decay values for Patagonian peatlands from our new peat cores and from published sites. Our model indicates that Holocene peat addition rates were significantly higher than what was reported for northern peatlands, but decay coefficients were similar between these two high-latitude regions. These results support the idea that long, mild, growing seasons promote peat formation in southern Patagonia. We presented the modern climate space of Patagonian peatlands (temperature, precipitation, and seasonality ranges). We found that Patagonian peatlands occupy a distinct climatological niche that corresponds to an end-member of the northern peatland climate domain, with a mild mean annual temperature (3-9 C) and very weak temperature seasonality. However climatic parameters and Holocene peat accumulation rates were not correlated, suggesting that autogenic factors might be controlling carbon sequestration in Patagonian peatlands. Second, we performed high-resolution plant macrofossil analysis to reconstruct peatland historical development. We found that most peatlands underwent a sudden vegetation shift from fen to bog around 4200 years ago, potentially due to increased precipitation associated with strengthening of the southern westerly winds and warmer summer temperatures associated with maximum insolation at that time. We also found that peatland development across the study region was characterized by repeated switches between wet- and dry-adapted plant communities during the 'bog' phase, depicting changes between hollows (wet assemblages) and lawns (dry assemblages) that could represent past surface patterning dynamics. The relationship between the frequency of the reconstructed wet-to-dry cycles and carbon accumulation rates followed a power function, such that rapid vegetation shifts were associated with high peat accumulation rates. These findings imply that, in systems characterized by a mosaic of alternative stable states, localized short-term instability may lead to long-term maintenance of ecosystem function such as carbon sequestration at the ecosystem scale. Overall, our observations provide a framework for addressing temporal changes of structural dynamics as controls on the carbon-sink capacity of terrestrial ecosystems. Third, we proposed to reconstruct past temperature and moisture conditions using stable isotopes and testate amoebae, respectively. We are in the process of generating a transfer function based on modern testate amoebae samples from Patagonia to link testate amoebae communities with peat moisture and water table depth. We have started to write up the results. Regarding stable isotopes, we have performed most of the laboratory analysis and are in the process of completing our time series and writing up the results. Lastly, recent studies have indicated that northern peatlands were rapidly accumulating large quantities of organic carbon during the early Holocene due to maximum incoming solar radiation and associated warm summer temperatures and strong seasonality. In light of these results, we hypothesized that Patagonian peatlands would show an increase in peat-carbon sequestration around 4000 years ago (southern hemisphere maximum solar insolation). Results from one of our sites lend support to this hypothesis, with a three-fold increase in PCAR around 4200 years ago that is simultaneous to the fen-to-bog transition. However, important changes in precipitation variability that were induced by changes in the strength and latitudinal position of the southern westerly winds have been reconstructed across the study region around this time. Such changes in effective moisture could have triggered the fen-to-bog transition and the associated increase in peat accumulation rate. Overall, the synchronicity between the maximum solar insolation and the changing regional precipitation regime 4200 years ago challenges our ability to test the hypothesis that the orbitally-induced temperature increase controlled peat-carbon sequestration in Patagonia, as observed in the northern hemisphere. Our ongoing paleoecological work (testate amoebae-inferred water table reconstructions and stable isotope-inferred temperature reconstructions) is needed to further test this hypothesis.
