Non-Traditional Stable Isotopes in Land-to-Sea Fluxes
Earth’s biogeochemical evolution is inferred from the elemental compositions of past seawater and the rock record. These reconstructions generally assume: 1) a known and relatively constant flux of continent-derived solutes through time, 2) that the isotopic composition of these input fluxes is constant over geologic timescales, and 3) that the sensitivity of isotope proxies is constant over geologic timescales. However, it is prudent to test these assumptions in the modern amidst a well-understood natural environment before applying them to reconstructions of the past, where far more unknowns exist. In Chapter 2, I quantified a previously unaccounted for input of continent-derived solutes: groundwater discharge. My results indicate that the global groundwater flux is responsible for 12 – 23% of the riverine flux for Li, Mg, Ca, Sr, and Ba. Moreover, only the isotopic compositions of δ7Li and δ138Ba were indistinguishable from riverine values. The δ26Mg, δ44Ca, 87Sr/86Sr and δ88/86Sr composition were all distinguishable from global riverine values. This difference was interpreted as an indication of the disproportionate role that coastal geology plays on the chemical composition of groundwater discharge. In chapter 3, I constructed the most comprehensive chemical mixing model of the Fraser River to-date, where the [Li], [Mg], [SO4], [K], [Ca], [Sr], δ7Li, δ26Mg, δ34S, δ41K, δ44Ca, 87Sr/86Sr, and δ88/86Sr composition of the dissolved load of the Fraser River was modeled and compared to observational data. The variations observed were consistent with (and able to be successfully modeled as) mixing between the hydrologic outputs of two end-member sources: young igneous rocks of the Coast Range and ancient (Paleozoic/Precambrian) metamorphosed sedimentary/carbonate bedrock of the Rocky Mountains. This result of Chapter 3 provides a first-order approximation of how these isotope systems may co-vary globally if surficial processes were to shift from a regime dominated by young, volcanic bedrock (e.g. trap volcanism) to a regime dominated by the erosion and weathering of collisional orogens dominated by ancient metasedimentary sequences (e.g. Himalayas). Furthermore, I found δ7Li to be the isotope system that correlated most significantly with silicate-derived cation fluxes (R2 = 0.2 – 0.4), which contributes to the growing body of work suggesting it be a more direct silicate weathering proxy than 87Sr/86Sr (Misra and Froelich, 2012). In Chapter 4, I focused on constraining the cycling and export of one of these non-traditional isotopes, δ138Ba, in Gulf of Aqaba seawater. Hot and devoid of riverine inputs, the Gulf of Aqaba has the lowest saturation state for barite of any marine basin in the world (Monnin et al., 1999), which I utilized as a modern analog for arid conditions of Earth’s past. My results indicate that the δ138Ba composition of the exported particulate flux does not vary with respect to regional parameters, such as barite saturation state and primary productivity. Furthermore, the results of this chapter bolster the argument that Δ138Ba, the isotopic offset between barite and seawater from which it precipitated (δ138Baparticulate - δ138Baseawater), is consistently -0.47‰. Overall, in this thesis, I have improved constraints on the magnitude and isotopic composition of groundwater, riverine, and exported particulate fluxes for modern seawater.