Holocene climate variability and associated paleoenvironmental changes in the eastern lowlands of Guatemala revealed by a lake sediment from Lake Izabal

<p>Sediment records have been widely used to reconstruct Holocene environmental and climate conditions around the world. As new Holocene records from Central America and the Caribbean have become available, new hypotheses have emerged to explain the complex hydroclimate variability in the region. Here we present results from a radiocarbon-dated sediment core recovered from Lake Izabal, eastern Guatemala, that covers the last ~9,500 years. We combined sedimentological, XRF elemental abundances, and principal component (PC) analyses to reconstruct changes in erosion/precipitation, lake productivity, and lake water chemistry during the Holocene. Our results indicate that during the early Holocene, Lake Izabal was a shallow lake with minimal catchment erosion/precipitation as indicated by the abundance of organic-rich mud, coupled with the lowest PC scores and titanium (Ti) abundance of the entire record. An overall increase in the PC scores and a progressive increase in Ti suggest that precipitation/erosion increased from 8,300 to 4,800 cal yr BP and remained high until 1,200 cal yr BP. There was then a significant reduction in erosion and precipitation at ca. 1,200 cal yr BP, as evidenced by a sharp decrease in magnetic susceptibility, terrigenic derived elements, and PC scores. We suggest that the transition towards wetter conditions from the early to the middle Holocene, followed by a stable wet climate until ca. 1,200 cal yr BP, was strongly influenced by a progressive increase in autumn insolation throughout the Holocene, which could have caused an increase in Caribbean sea surface temperatures, increasing moisture availability leading to greater precipitation amounts in the Caribbean coast of Central America.</p>

Towards reconstructing Paleo-water activity using lake level and gastropod TE/Ca ratios

<p>Water activity is the partial pressure of water in a solution, which is a crucial driver of chemical (e.g. dissociation constants) and physical (e.g. evaporation and stable isotope enrichment) properties of water. As such, reconstructing past climate and environmental conditions using lacustrine proxies requires a quantification of past water activity. However, very little research has been conducted on ways to reconstruct this crucial water property. In this research, we present a novel method that potentially can enable reconstructing paleo water activity in closed basin lakes. Closed-basin lakes are lakes with no outlet; thus, the size of the lakes varies as function precipitation and evaporation (P-E), and in turn, can be used to quantitatively reconstruct these conditions. Past climate change altered P-E throughout the geological history causing lakes all around the world to expand (contract), which in turn diluted (concentrated) and reduced (enhanced) lake water activity. To fully understand past hydrological and chemical processes in the lake, it is crucial to quantify the water activity in the lake.</p><p>radix sp. is a gastropod that lives at the margins of closed basin lakes. It forms a hard CaCO<sub>3</sub> shell, which at death is deposited in the sediments, remains in the geological record, and informs on past lake levels. We sampled radix sp. shells in elevated shorelines of a closed basin lake in North China and from the modern lake shoreline. Radiocarbon ages of the shells span the past 16 ka and are used to construct the lake level curve for the Late Glacial to Holocene (Goldsmith et al., 2017). During this time period the lake level and volume changed substantially; the Early – Mid Holocene lake area high-stand was x7 larger than the modern lake. On these gastropod shells we measured high resolution profiles (sample/length) of TE/Ca ratios (e.g. Mg/Ca, Sr/Ca) using Laser Ablation ICP-MS. We calculated the dissociation constants of the different TE/Ca ratios using modern shells and modern lake water chemistry.</p><p>Our preliminary results show that of all elements analysed, Mg/Ca ratios best track lake volume and therefore can be used to reconstruct lake level. The next step will be using gastropod derived lake water chemistry to reconstruct past water activity. This method entails comparing the TE/Ca ratios of Late Glacial and Holocene gastropods with a hydro-chemical model of the lake, derived from a volume history reconstruction of the lake. The hydro-chemical model assumes a simple addition of river water to the modern lake, until the lake reaches the elevation that each gastropod was sampled at. In tandem, these two data sets constrain water chemistry, and the difference between these two estimates should represent the effect changes in water activity have on the disassociation constant between lake water and shell TE/Ca ratios.</p><p>Goldsmith, Y., Broecker, W.S., Xu, H., Polissar, P.J., DeMenocal, P.B., Porat, N., Lan, J., Cheng, P., Zhou, W., An, Z., 2017. Northward extent of East Asian monsoon covaries with intensity on orbital and millennial timescales. Proc. Natl. Acad. Sci. U. S. A. 114, 1817–1821.</p>

A Series of Data-Driven Hypotheses for Inferring Biogeochemical Conditions in Alkaline Lakes and Their Deposits Based on the Behavior of Mg and SiO2

Alkaline (pH > 8.5) lakes have been common features of Earth’s surface environments throughout its history and are currently among the most biologically productive environments on the planet. The chemistry of alkaline lakes favors the deposition of aluminum-poor magnesian clays (e.g., sepiolite, stevensite, and kerolite) whose chemistry and mineralogy may provide a useful record of the biogeochemistry of the lake waters from which they were precipitated. In this forward-looking review, we present six data-driven, testable hypotheses devoted to furthering our understanding of the biogeochemical conditions in paleolake waters based on the geochemical behavior of Mg and SiO2. In the development of these hypotheses, we bring together a compilation of modern lake water chemistry, recently published and new experimental data, and empirical, thermodynamic, and kinetic relationships developed from these data. We subdivide the hypotheses and supporting evidence into three categories: (1) interpreting paleolake chemistry from mineralogy; (2) interpreting the impact of diatoms on alkaline lake sedimentation; and (3) interpreting depositional mineralogy based on water chemistry. We demonstrate the need for further investigation by discussing evidence both for and against each hypothesis, which, in turn, highlights the gaps in our knowledge and the importance of furthering our understanding of the relevant geological and biological systems. The focused testing of these hypotheses against modern occurrences and the geologic record of alkaline lakes can have profound implications for the interpretation of the paleo-biogeochemistry and paleohabitability of these systems on Earth and beyond.

The Variability of Lake Water Chemistry in the Bory Tucholskie National Park (Northern Poland)

The paper presents the results of chemical analysis of lake waters in Bory Tucholskie National Park (BTNP). The BTNP area is unique due to its location within a single catchment and high variability in geological structure. Moreover, the lakes have different morphometric parameters, represent different hydrological types, trophic types and thermal regimes. Another unique feature is the existence of five lobelia lakes. This name comes from the Latin name of the taxon – Lobelia dortmanna L. which has been included in the Polish Red Data Book of Plants. The chemical analysis included 55 parameters, within macro elements (MEs), trace elements (TEs) and rare earth elements (REEs). Low concentrations of MEs, TEs and REEs confirm the absence of anthropogenic pressure. High variation of ME, TE and REE contents between individual lakes is due to different geological structure. The cluster analysis enabled lakes to be divided into six groups taking into account all analyzed water quality parameters. The lobelia lakes were characterized by the lowest concentrations of MEs and REEs, which mainly result from the small catchment area and their mainly endorheic character. The highest variability of MEs, TEs and REEs occurred in endorheic lakes, where the geological structure was dominant. The lowest variability of MEs, TEs and REEs occurred in the lakes connected by the Struga Siedmiu Jezior stream. The analysis of MEs, TEs and REEs in relation to the environmental factors and trophic, hydrologic and thermal typology allowed a better understanding of their spatial distribution in the BTNP lakes. The obtained results indicate that the values of the studied elements were generally close to the average values noted in surface waters according to the Geochemical Atlas of Europe.

When “evaporites” are not formed by evaporation: The role of temperature and pCO2 on saline deposits of the Eocene Green River Formation, Colorado, USA

Halite precipitates in the Dead Sea during winter but re-dissolves above the thermocline upon summer warming, “focusing” halite deposition below the thermocline (Sirota et al., 2016, 2017, 2018). Here we develop an “evaporite focusing” model for evaporites (nahcolite + halite) preserved in a restricted area of the Eocene Green River Formation in the Piceance Creek Basin of Colorado, USA. Nahcolite solubility is dependent on partial pressure of carbon dioxide (pCO2) as well as temperature (T), so these models covary with both T and pCO2. In the lake that filled the Piceance Creek Basin, halite, nahcolite or mixtures of both could have precipitated during winter cooling, depending on the CO2 content in different parts of the lake. Preservation of these minerals occurs below the thermocline (>∼25 m) in deeper portions of the basin. Our modeling addresses both: (1) the restriction of evaporites in the Piceance Creek Basin to the center of the basin without recourse to later dissolution and (2) the variable mineralogy of the evaporites without recourse to changes in lake water chemistry. T from 20 to 30 °C and pCO2 between 1800 and 2800 ppm are reasonable estimates for the conditions in the Piceance Creek Basin paleolake. Other evaporites occur in the center of basins but do not extend out to the edges of the basin. Evaporite focusing caused by summer-winter T changes in the solubility of the minerals should be considered for such deposits and variable pCO2 within the evaporating brines also needs to be considered if pCO2 sensitive minerals are found.

Landscape and groundwater controls over boreal lake water chemistry and water balance heterogeneity in an esker complex of northeastern Ontario, Canada

Water chemistry and water isotopic composition were investigated in a set of 50 boreal lakes located at different elevations in an esker system near Timmins, Ontario, as well as in local streams, groundwater springs and information available from seasonal precipitation values. Analyses focused on stable isotopic ratios of hydrogen and oxygen, as well as specific conductance as indicators of the position of a lake with respect to the influence of groundwater. Both isotopic composition and specific conductance distinguished higher elevation groundwater discharge lakes from lower elevation groundwater recharge lakes. Groundwater recharge lakes characterized by enriched isotopic values and low values of specific conductance are located above the hydraulic midline elevation of the study lakes. In contrast groundwater discharge lakes, were isotopically depleted and had higher values of specific conductance, and occurred below the hydraulic midline of the study lakes. An intermediate group of lakes was also defined (termed seepage lakes) and consisted of either recharge lakes that were alkaline, or discharge lakes that had no outlet. The seepage lakes group had intermediate isotopic and water chemistry characteristics compared to recharge and discharge lakes. A classification scheme for lakes was developed based on the specific conductivity, water isotopic composition, the presence of an outlet, and other characteristic to define three types of recharge lakes, and two types of discharge lakes. Interannual (2013 and 2014) and seasonal differences in water chemistry between (early June and August) revealed that upland groundwater recharge lakes showed evidence of evaporative drawdown, indicating sensitivity to short-term changes in climate, whereas the lower-elevation discharge showed little variation between seasonal samples, and consequently would be affected by hydroclimatological changes of greater duration and persistence.