Sr isotopes in the Tortonian-Messinian Lake Bira and Gesher marshes, Northern Valleys of Israel: Implications for hydroclimate changes in East Mediterranean−Levant margins

87Sr/86Sr isotope and Sr/Ca ratios in lacustrine carbonates were used to reconstruct the hydroclimate conditions in the watershed of Lake Bira that filled during the Tortonian-Messinian the tectonic depressions of the Northern Valleys of Israel in the East Mediterranean-Levant region. 87Sr/86Sr ratios of the Tortonian (ca. 10−8 Ma) carbonates of ∼0.7075 and the great expansion of the lake indicate wet conditions and enhanced supply of freshwater from the regional Mesozoic aquifers. Upon the transition to the Messinian period (ca. 7−6 Ma), the 87Sr/86Sr ratios in the carbonates rose to ∼0.7080−0.7085, reflecting the contribution of Sr from Sahara Desert dusts that came to comprise the regional surface cover. This contribution is also reflected in the silicate fraction of the lacustrine formations that show “granitic-crustal” 87Sr/86Sr ratios of ∼0.711. During the Messinian salinity crisis (5.9−5.6 Ma), the region became arid and Lake Bira possibly dried. Later, during the Lago Mare stage (ca. 5.5−5.3 Ma), the rainfall increased and paludal waterbodies scattered the area of the larger Lake Bira.

Rise of the Colorado Plateau: A Synthesis of Paleoelevation Constraints From the Region and a Path Forward Using Temperature-Based Elevation Proxies

The Colorado Plateau’s complex landscape has motivated over a century of debate, key to which is understanding the timing and processes of surface uplift of the greater Colorado Plateau region, and its interactions with erosion, drainage reorganization, and landscape evolution. Here, we evaluate what is known about the surface uplift history from prior paleoelevation estimates from the region by synthesizing and evaluating estimates 1) in context inferred from geologic, geomorphic, and thermochronologic constraints, and 2) in light of recent isotopic and paleobotanical proxy method advancements. Altogether, existing data and estimates suggest that half-modern surface elevations were attained by the end of the Laramide orogeny (∼40 Ma), and near-modern surface elevations by the mid-Miocene (∼16 Ma). However, our analysis of paleoelevation proxy methods highlights the need to improve proxy estimates from carbonate and floral archives including the ∼6–16 Ma Bidahochi and ∼34 Ma Florissant Formations and explore understudied (with respect to paleoelevation) Laramide basin deposits to fill knowledge gaps. We argue that there are opportunities to leverage recent advancements in temperature-based paleoaltimetry to refine the surface uplift history; for instance, via systematic comparison of clumped isotope and paleobotanical thermometry methods applied to lacustrine carbonates that span the region in both space and time, and by use of paleoclimate model mediated lapse rates in paleoelevation reconstruction.

Coeval primary and diagenetic carbonates in lacustrine sediments challenge palaeoclimate interpretations

Lakes are sensitive to climate change and their sediments play a pivotal role as environmental recorders. The oxygen and carbon isotope composition (δ18O and δ13C) of carbonates from alkaline lakes is featured in numerous studies attempting a quantitative reconstruction of rainfall, temperature and precipitation-evaporation changes. An often-overlooked challenge consists in the mineralogically mixed nature of carbonates themselves. We document a large variability of carbonate components and their respective distinct δ18O and δ13C values from sediments of Lake Van (Turkey) covering the last 150 kyr. The carbonate inventory consists of primary (1) inorganic calcite and aragonite precipitating in the surface-water, (2) biogenic calcite ostracod valves; and post-depositional phases: (3) dolomite forming in the sediment, and previously overlooked, (4) aragonite encrustations formed rapidly around decaying organic matter. We find a systematic relation between the lithology and the dominant deep-water carbonate phase formed recurrently under specific hydrological conditions. The presence of the different carbonates is never mutually exclusive, and the isotopic composition of each phase forms a distinctive cluster characteristic for the depth and timing of their formation. Our findings stretch the envelope of mechanisms forming lacustrine carbonates and highlight the urge to identify and separate carbonate components prior to geochemical analyses.

Tectonically conditioned record of continental interior paleoclimate during the Carnian Pluvial Episode: The Upper Triassic Los Rastros Formation, Argentina

Discerning paleoclimate parameters in depositional systems of the continental interior is challenging because the system response and stratigraphic record of climate are controlled by tectonic processes and are mediated through landscape and hydrological evolution of fluvial lacustrine systems. Climate and tectonic signals cannot be deconvolved from stratigraphic patterns alone but require additional information or data sets that directly record climate or tectonic influence. The Carnian Los Rastros Formation in northwest Argentina provides an excellent case study that integrates an appropriate range of information in a system with strong climate and tectonic signals, being deposited in part during the Carnian Pluvial Episode and spanning the active rift phase of the Ischigualasto−Villa Unión Basin. We examined the stratigraphic and spatial patterns of carbon (C) and oxygen (O) stable isotopes in lacustrine carbonates from the Los Rastros Formation in multiple parts of the basin to constrain paleohydrological conditions and paleotemperatures. Practically all C and O isotope values are characterized by negative values: δ18Ocarb −11.6‰ and −15.7‰ (χ average −13.1‰; 1σ = 1.6) and δ13Ccarb −2.6‰ to −8.0‰ (χ average −5.1‰; 1σ = 2.1), reflecting the latitude, altitude, and continentality of the lake system and its vegetated and humid catchment area. Stratigraphic patterns of stable isotope data from two different localities (Cerro Bola North and Cerro Bola South) show a change from short water-residence time to long residence time and back to short residence time. This contrasts with sedimentologic, organic geochemical, and small-scale stratigraphic patterns that indicate an overfilled lake basin, which is expected to contain a completely open-hydrology isotopic signature. Paleotemperatures calculated from marginal lacustrine carbonates show a warm and quite variable paleothermal range consonant with their continental interior position and with Global Climate Model estimates for high paleolatitudes. Warmer paleotemperatures (linked to aridity, probably smaller lake size, and less thermal mass) precede the Carnian Pluvial Episode, whereas relatively cooler paleotemperatures coincide with the Carnian Pluvial Episode (linked to humidity, probably larger lake size, and more thermal mass). Carbon and oxygen stable isotope signatures integrated with sedimentologic and physiographic information allow us to propose that tectonics, specifically, half-graben tilting during the active synrift phase, dominated over climate effects as the cause of hydrological fluctuations of this system, even during the Carnian Pluvial Episode. Without appropriate stratigraphic-tectonic context, single-proxy reconstructions of continental-interior paleoclimate can be misleading. A robust interpretation of climate effects requires characterization of tectonic effects, geomorphology, paleohydrology, and sedimentary system responses.

How (not) to quantify hydrological changes using lacustrine carbonates?

<p>Lakes are sensitive to climate change and their sedimentary components play a pivotal role as environmental recorders. In the past, lacustrine carbonates have been utilized in a number of studies attempting at a quantitative reconstruction of rainfall and/or precipitation-evaporation changes based on the biogenic or bulk carbonate δ<sup>18</sup>O signature. While these studies are built on sound theoretical grounds of mass balance and kinetic isotopic fractionation, the challenge often overlooked is the mineralogically mixed nature of carbonates comprising the bulk.</p><p>We report a case study from Lake Van, the world’s largest alkaline lake. Our time series comprising ca. 140 ka documents not only changing proportions of surface water calcite and aragonite, but also diagenetic bottom-water dolomite and, for the first time in Lake Van, early diagenetic bottom-water aragonite. Importantly, in the Lake Van profile primary and early diagenetic carbonates (in particular aragonite) are concurrent rather than mutually exclusive. A comprehensive comparison of the δ<sup>18</sup>O and δ<sup>13</sup>C compositions of singled out water column, biogenic (ostracod valves) and diagenetic carbonates shows, that each of the fractions forms a distinctive cluster characteristic for the depth and timing of their formation. Also, the differences between δ<sup>18</sup>O values of concurrent deep-water carbonate phases exceed what is expected from mineral-specific fractionation. Our data suggest that, an uncritical and unchecked application of the isotopic composition of the bulk carbonate fraction in quantitative climate reconstructions can severely compromise the results. We also advocate that, among different carbonate fractions in Lake Van, monospecific biogenic samples most faithfully reflect the oxygen isotopic composition of the lake water contemporaneous to their deposition, while the carbon composition of biogenic samples is additionally influenced by the organism microhabitat.</p>