What are global empirical calibrations hiding? The case of brGDGT membrane lipids
Abstract
To allow quantitative reconstruction of climate change, distributions of lipids conserved in geological archives have been used to estimate past climate changes. BrGDGTs, membrane lipids of unknown bacteria, are at the basis of the commonly used climate proxies MBT'5ME and CBT', used to reconstruct mean annual temperature (MAT) and soil pH, respectively. The calibrations of these climate proxies are based on modern environmental datasets that span large spatial and environmental ranges. However, these calibrations are empirical, and the mechanism causing the observed relationship is not known. It is therefore not clear whether this global dependency really reflects the effect of local climate changes.
To determine whether the environmental dependency of brGDGTs reflects a physiological plasticity of soil bacteria, or whether they reflect the replacement of different communities of brGDGT producers, we combine brGDGT and 16S rDNA data. We use this approach at two sites with strong environmental (temperature, pH) gradients on a small spatial scale, caused by below-ground geothermal warming (ForHot, Iceland) or long-term fertilization treatments (Ossekampen experimental site, The Netherlands). Non-linear changes in brGDGT distributions are observed at both sites, coinciding with shifts in the bacterial community composition. Rather than reflecting a pure biochemical response, the brGDGT signal produced in soils thus probably reflects a phylogenetic signal. Projecting this local threshold behavior on the global soil data set generates 3 separate soil clusters with a distinct brGDGTs fingerprint. Within each cluster brGDGTs show different dependencies with MAT and soil pH. The environmental dependency of the MBT'5ME differs between some soil clusters, and is strongly affected when the environmental thresholds that separate the clusters are crossed. The global empirical temperature calibration is thus hiding several threshold values, that can influence MBT'5ME values in sometimes non-intuitive ways. This complicates the interpretation of brGDGT-based paleoclimate records. As the accuracy of climate reconstructions depends on well-understood climate proxies, elucidating the intricacies of the MBT'5ME climate proxy is an important step in developing a reliable proxy for soil temperature.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP23A..03D
- Keywords:
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- 0424 Biosignatures and proxies;
- BIOGEOSCIENCESDE: 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 4912 Biogeochemical cycles;
- processes;
- and modeling;
- PALEOCEANOGRAPHYDE: 4924 Geochemical tracers;
- PALEOCEANOGRAPHY