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Geometric and Spectral Analysis of Relict Channel Planforms in Central Baja California, Mexico: A Novel Approach to Paleo-Sea Level Reconstruction and Testing Hypotheses for Genetic DivergenceGenetic divergence along the central Baja California Peninsula, Mexico, has been hypothesized to reflect a Pliocene cross-peninsular seaway that previously isolated northern and southern populations of terrestrial plants and animals. One way to test this hypothesis is through quantitative analysis of relict channels preserved on low-relief paleo-surfaces. Recognition of tidal channels on relict landscapes offers a powerful tool for reconstructing past sea level in tectonically active arid coastal regions where crustal uplift results in relative sea-level fall and preservation of ancient channel networks. This method requires reliable criteria to distinguish fluvial versus tidal channels, which is challenging due to the overlap of standard metrics for the two channel types, and possible inheritance or overprinting of geometries. We improve the utility of existing metrics and explore the potential for identifying paleo-sea-level indicators by analyzing modern and ancient channels to identify unique patterns in planform geometry and to evaluate their applicability for classifying tidal versus fluvial origins. Preliminary measurements of geographically diverse modern systems reveal distinct, quantifiable differences between the two channel types in along-channel curvature, width, and wavelet spectra. Modern tidal channels display a pronounced and systematic down-channel increase in channel width and decrease in curvature. In contrast, modern fluvial channelsmore »
Genetic divergence and ephemeral barriers: Reconciling genetic and geological timescales within geogenomicsUnderstanding the timescales on which different geologic processes influence genetic divergence is crucial to defining and testing geogenomic hypotheses and characterizing Earth- life evolution. To see if we can recover a genetic signal produced by a hypothetical physical barrier to gene flow, we used a geographically explicit simulation approach. We used the CDMetaPop software to simulate heritable genetic, nonadaptive, data for 20 geographically distinct populations distributed throughout the Baja California peninsula of Mexico, a landscape where a transpeninsular seaway barrier has been proposed to have isolated the southern peninsula and caused the observed latitudinal genetic divergence in over 80 terrestrial species. We simulated 10,000 generations of isolation by a barrier under two dispersal scenarios (1 km and 100 km of max. dispersal from population of origin per generation) and three DNA substitution rates (10-7, 10-8 and 10-9 nucleotide substitutions per site per generation). Our simulations indicate that a physical barrier can produce strong genetic divergence within 10,000 generations, comparable to the continuum of values observed in nature for different taxonomic groups and geological settings. We found that the generation time of the organism was by far the most important factor dictating the rate of divergence. Evaluating different generation times (0.02,more »
Geochronological Constraints on the Volcanic and Topographic Evolution of Central Baja California, MexicoCentral Baja California (BC) experienced tectonism and volcanism that shaped the landscape from the Miocene to Recent. One important feature is the San Ignacio trough (SIT) that hosted a marine seaway or embayment and acted as a physical barrier to animal and plant migration. This barrier may be responsible for a well-known break in the DNA, N and S of this region. Central BC has also hosted contemporary voluminous and chemically diverse volcanism. Radiometric ages provide important constraints on the origins and longevity of critical topographic features. The Baja GeoGenomics research group is investigating the nature and timing of Pliocene marine and tidal deposits in the NE-oriented, low-lying SIT, located W of the peninsular divide. These new data reveal that the Sierra San Francisco, a highland volcanic area immediately N of the SIT, is a series of volcanoes constructed of dacitic and andesitic Peleean domes with voluminous lahar and pyroclastic flow deposits. These calcalkaline rocks were previously thought to be subduction-related magmatism and part of the early to middle Miocene (~2412 Ma) Comondu Group. However, zircon U-Pb and 40Ar/39Ar dates yield ages of 11-9 Ma. These data indicate the Sierra San Francisco erupted post-subduction and is not part of themore »
Volcano-Tectonic Evolution of Central Baja California Peninsula, Mexico: Implications for Speciation and Barriers to Gene FlowLate Cenozoic evolution of the Baja California (BC) peninsula governs its species diversity, with changes to terrestrial habitats and shorelines driven by volcanic and tectonic processes. New geologic mapping and geochronology in central BC help assess if recent landscape evolution created a barrier to gene flow. The NW-trending topographic divide of the BC peninsula near San Ignacio-Santa Rosalia (27.4N) is a low (400500 m asl), broad (2030 km-wide) pass. At the pass, ~2022-Ma volcaniclastic strata, mafic lavas, fluvial conglomerate, cross-bedded eolian sandstone, and a felsic tuff dip ~515 SW. Similar lithology and chronology suggest these strata correlate to the lower Comondu Group (CG). They are overlain by middle Miocene (~1114 Ma) mafic lavas with similar SW dips that overlap in age with the upper CG. NW of the pass, upper Miocene (~9.511 Ma) post-CG volcaniclastic strata and mafic lava flows are exposed in the Sierra San Francisco and dip ~10 SE on its SE flank, inclined differently than older SW-dipping CG at the pass. The basalt of Esperanza (~10 Ma) unconformably overlies the CG at and west of the pass. Its ~1 regional dip suggests that ~515 of SW tilting occurred prior to ~10 Ma in the footwall of themore »
Cryogenic spectrometer for measuring the far-IR to millimeter-wave absorptivity of cosmic analog dusts
We report on the design, construction, and performance of a custom apparatus built to measure the frequency- and temperature-dependent absorptivity of millimeter-wave light by cosmic analog dusts. We highlight the unique challenges faced as well as a few key innovations that are part of the instrument. Among those is an ultra-compact Fourier transform spectrometer. We have measured its effective frequency range and FWHM resolution to be 150–2100 GHz and
, respectively. Another innovation is a cold sample positioner whose temperature can be controlled within the range of 3.7–50 K. The use of a pulse-tube cryocooler results in a pulse-synchronous signal that dominates the detector (bolometer) signal. Methods used to address that challenge are also presented.