STRATIGRAPHIC DEPENDENCE OF CHLORINE ISOTOPE VALUES IN GALE CRATER
New analyses sampling bedrock and a sand dune have been acquired as Curiosity ascends Mount Sharp. These new data reveal a statistically significant correlation between stratigraphic height and δ37Cl in the fluvio-lacustrine sedimentary rocks above the Bradbury landing site. Starting in the Kimberly formation (-58±3‰), rising through the Murray formation (-49±8‰ to -31±9‰) and ending in the altered Stimson unit (-25±5‰), a monotonic relationship between elevation and δ37Cl is observed.
We are presently seeking a cause for this correlation. Long-term temporal variations in instrument response are considered because the rover has largely driven up-section, so stratigraphic height correlates crudely with time. However, the instrumental drift scenario may be ruled out by multiple lines of evidence including: 1) duplicate δ37Cl measurements separated by 533 sols on a sample stored on board the rover are repeatable within error; 2) samples outside of the fluvio-lacustrine stratigraphy (one rock and two sediment samples) measured contemporaneously with those in the sequence do not fit the trend. We suspect the δ37Cl correlation with stratigraphic height is a primary feature of the rock, and not due to post-depositional alteration. A correlation between δ37Cl and Cl or O2 would imply that the isotopic ratio is determined by the relative amounts of oxychlorine and chloride phases, making the trend a simple mixing relationship, but these values do not correlate. Comparing δ37Cl results to other data acquired by Curiosity, we observe no compelling correlations with chemistry or mineralogy, although weak correlations with K2O, Zn, and SiO2/TiO2 may exist; the extent and implications of these correlations of this are being explored.