A Tufa Nut to Crack: The Timing and Duration of Lacustrine Tufa Formation in the Late Pleistocene
Abstract
In arid regions worldwide, extensive build-ups of porous carbonate rock called "tufas" are unmistakable evidence for past landscape occupation by pluvial lakes. These tufas frequently exhibit a rich diversity of architectural structure, morphology, composition, and texture, but leveraging these characteristics to reconstruct the conditions and processes responsible for this diversity is often limited by uncertainties in the timing and rate of tufa formation. Conversely, past attempts at dating tufas have been confounded by poor understanding of the complex makeup of these deposits, leading to misguided conclusions on both the utility of certain geochronological tools for tufas and their age.
In this presentation, we ask: Precisely when and at what rate did tufas form? We present data from late Pleistocene lake basins in the central Andes and Searles Lake in the western U.S. for insight. We demonstrate the descriptive power of combining (1) precise uranium-thorium (U-Th) dates on tufas and other lake carbonates with (2) geologic observations of their depositional context at all scales - from the outcrop to the microscale. These analyses inform one another: Taking advantage of our ability to U-Th date small (<10 mg) amounts of powder, we use petrography to improve sample selection, and then test data for internal consistency by applying physical stratigraphic and coevality constraints. From this, we observe that U-Th dates on dense (non-porous) or crystalline materials more often yield data that pass these tests. We also document open system behavior in deposits that would nominally pass traditional geochemical criteria for valid U-Th dates. Pairing quality-controlled U-Th dates with outcrop and petrographic observations, we present more nuanced insights on the timing and variability of past lake levels associated with tufa formation. Despite these geochronological challenges, many observations suggest that some tufa varieties may have formed relatively rapidly (<100 years): carbonate coatings of gas bubbles and algal filaments; casts of tree bark and brine fly pupae casings; and preservation of cyanobacterial sheaths. As the diversity of tufa expressions implies, different tufa varieties in the same basin likely represent a wide range of formation rates and thus record snapshots of varying length in time.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMPP34B..03C
- Keywords:
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- 1039 Alteration and weathering processes;
- GEOCHEMISTRY;
- 1051 Sedimentary geochemistry;
- GEOCHEMISTRY;
- 4217 Coastal processes;
- OCEANOGRAPHY: GENERAL;
- 4912 Biogeochemical cycles;
- processes;
- and modeling;
- PALEOCEANOGRAPHY