Open-basin Lakes Research Articles

Spatiotemporal variation of modern lake, stream, and soil water isotopes in Iceland

Abstract. As global warming progresses, changes in high-latitude precipitation are expected to impart long-lasting impacts on Earth systems, including glacier mass balance and ecosystem structures. Reconstructing past changes in high-latitude precipitation and hydroclimate from networks of continuous lake records offers one way to improve forecasts of precipitation and precipitation–evaporation balances, but these reconstructions are currently hindered by the incomplete understanding of controls on lake and soil water isotopes. Here, we study the distribution of modern water isotopes in Icelandic lakes, streams, and surface soils collected in 2002, 2003, 2004, 2014, 2019, and 2020 to understand the geographic, geomorphic, and environmental controls on their regional and interannual variability. We find that lake water isotopes in open-basin (through-flowing) lakes reflect local precipitation, with biases toward the cold season, particularly in lakes with sub-annual residence times. Closed-basin lakes have water isotope and deuterium excess values consistent with evaporative enrichment. Interannual and seasonal variabilities of lake water isotopes at repeatedly sampled sites are consistent with instrumental records of winter snowfall; summer relative humidity; and atmospheric circulation patterns, such as the North Atlantic Oscillation. Summer surface soil water isotopes span the entire range of seasonal precipitation values in Iceland and appear to be consistently overprinted by evaporative enrichment, which can occur throughout the year, although the sampling depths were shallower than rooting depths for many plant types. This dataset provides new insight into the functionality of water isotopes in Icelandic environments and offers renewed possibilities for optimized site selection and proxy interpretation in future paleohydrological studies on this North Atlantic outpost.

Holocene hydroclimate variability of the Baltic region inferred from stable isotopes, d-excess and multi-proxy data at lake Nuudsaku, Estonia (NE Europe)

Long-term hydroclimate records provide an opportunity to understand potential drivers of the past, and give context to modern and future climate warming. A wide variety of proxy data now allow for reconstruction of climate variables that were not previously possible. Here we present a multi-proxy dataset including n-alkane δ2H (δ2Hn-alk) values from an open-basin lake in Estonia to reconstruct past hydroclimate conditions for the eastern Baltic region. We complement our sedimentary δ2Hn-alk data with existing carbonate-based oxygen stable isotope (δ18O) data to derive deuterium (d-) excess. We present multiple isotopic records and reconstructed relative humidity (ΔRH) values over the Holocene, and link these with modern precipitation δ2H and δ18O values to guide the interpretation of the paleo-proxies. Fossil pollen and chironomid-based temperature reconstructions, as well as biogeochemical data provide additional information for inferring past environmental changes. Our results indicate that the middle Holocene in Estonia had on average 6 ± 3% higher RH values than the late Holocene. The δ18O and δ2H values were also higher during the middle Holocene, which we interpret as increased warm season precipitation. Our reconstructed d-excess values were relatively higher during the middle Holocene, indicating a more northerly or cold source water origin, in comparison to the late Holocene. In addition to the paleoclimatic significance, our results show how multiple quantitative proxies can be combined to characterize hydroclimate sensitivity to changes in relative humidity, temperature and moisture source.

Detection of aqueous alteration minerals in Martian open and closed paleolake basins

Candidate closed- and open-basin lakes have been widely observed on the Martian surface. Many of them show the presence of exposed sediments of possible lacustrine origin. Understanding the mineralogy of these structures provide insights on the nature of processes that originated and shaped their interior and on the role of liquid water on Mars. Spectral data obtained from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) have been so far analyzed with particular attention to the possible detection of aqueous alteration minerals. In this work, we mapped 16 closed-basins and 39 open-basins, not present in the past published Martian basins maps. We looked for recent CRISM observations of these and other previously mapped open- and closed-basin lakes. We further confirmed the presence of water alteration minerals in two previously analyzed structures and discovered other deposits of such minerals in two of the newly mapped sites. Light-toned bedrock exposures that show the presence of hydrated minerals (such as Fe/Mg smectites) along with unaltered mafic material (olivine) have been observed. Based on our observations and the comparison with the literature, it can be deduced that the observed deposits are most probably of lacustrine origin. Furthermore, the location of these deposits of altered minerals suggests that, at least in one case (where altered material has been detected on the floor of the outlet valley), the observed minerals have likely been transported within the lacustrine deposits rather than altered in situ. To the best of our knowledge, this is the first time that this occurrence has been observed and it is an indication of presence of such kind of minerals even in small valleys.

Precipitation and aridity constraints from paleolakes on early Mars

AbstractThe ancient climate of Mars remains an enigma despite the abundance of in situ and remote-sensing data revealing hydrological activity in the past. The crux of this debate—informed by geomorphic studies, chemical alteration observations, and numerical climate models—is the amount, distribution, and nature of surface water and precipitation (i.e., rain vs. snow). Here, we combined measurements of 96 open- and closed-basin lakes with simple hydrological balances to constrain catchment-averaged precipitation, PT, integrated over a given runoff episode of indeterminate length during early Mars history. We included 13 systems containing both open- and closed-basin lakes, providing fully bounded precipitation estimates (coupled systems). We show that, on average, PT was ≳4 m and ≲159 m; however, local precipitation was spatially variable and consistent with a complex, planetwide climate. Aridity indexes of open-basin lakes indicate some regions may have been at least as humid as semiarid terrestrial environments. Our results offer widely distributed, quantitative hydro-climate constraints that can be used to test paleoclimate model scenarios, working toward bridging the gap between geological observations and climate theory for early Mars.

Lake highstands in the northern Kalahari, Botswana, during MIS 3b and LGM

The interpretations of late Quaternary climate variation in the Kalahari are controversial. Only few speleothem humidity records do exist and reconstruction of aridity periods using aeolian sediments is challenged by considerable dating uncertainties. On the other hand studies on the large structural lake basins of the Kalahari have to cope with tectonics and the active catchment being far in the north, in a different climate zone. Thus hydrological signals from such palaeolakes may not or only partly reflect Kalahari palaeoclimate. The sediments of a regional drainage basin, west of the Tsodilo Hills, northern Kalahari, archive hydrological signals not superimposed by remote river catchment and tectonics. A ca. 3 m thick sediment profile near the palaeoshore of the former lake exhibits two lake highstands dated to MIS 3b and LGM. Geomorphological studies, fossil aquatic assemblages, their comparison to modern analogues and geochemical analyses, including sclerochronological stable isotope patterns from gastropod shells, suggest relatively deep, freshwater to oligohaline, open-basin lake periods. The data indicates that the climate was more humid than today, with two intra-annual rainfall maxima during the LGM, but likely under temperature regimes similar to modern times. The lake highstands were probably in phase with an inter-hemispheric Atlantic climate seesaw, which may imply that the main moisture source was rather the southern Atlantic than the Indian Ocean.

Cladoceran assemblage distribution in shallow alpine lakes of Sierra Nevada (Spain) and its relationship with environmental variables

Subfossil cladoceran assemblages were studied in surface sediments from 17 shallow alpine lakes in the Sierra Nevada (Spain) to evaluate relationships between species assemblages and environmental variables. Factors associated with the presence of Daphnia were also investigated. Chydorids predominated in the cladoceran assemblages. Redundancy analyses (RDA) revealed that the distribution of cladoceran assemblages was mainly associated with morphometric variables (extent of catchment vegetation in basin, lake depth and size, and the presence of inlets and outlets in basin). The explanatory power of chemical parameters was lower, with total nitrogen (TN) being the most influential variable. The relative abundances of Alona quadrangularis and Chydorus sphaericus showed a significant negative relationship in the lake set, suggesting the adaptation of these species to diverging lakes conditions in Sierra Nevada. More favorable conditions for species growth (warmer waters and increased input of material from the catchment in small, open-basin lakes surrounded by alpine meadows) were observed in lakes with higher abundance of A. quadrangularis than in those with higher abundance of C. sphaericus. Our results agree with those from previous paleolimnological studies in Sierra Nevada which show an abrupt increase in the relative abundance of A. quadrangularis at the expense of C. sphaericus since 1970 associated with more favorable conditions for growth as a result of climate change. Positive relationships were also observed between the relative abundance of Daphnia pulex gr. and closed, deep basins.

Radar Sounding of Open Basin Lakes on Mars

AbstractOrbital observations of the surface of Mars indicate that ancient basins were once host to lakes that may have been depocenters of sedimentary materials, including hydrated minerals like phyllosilicates. Later volcanic and sedimentary resurfacing may have developed a stratigraphy within the basins detectable through radar sounding data from the Shallow Radar instrument. Our radar survey of 61 open basin lakes (OBLs) revealed only one plausible reflector in a single basin east of Hellas Planitia. We investigated possible factors contributing to lack of radar detections in the other basins through detailed analysis of radargrams and subsurface characteristics of OBLs. As possible hosts to previous habitable environments, OBLs on Mars are important sites for future robotic and human missions. A full understanding of the factors influencing the radar signal, as addressed here, is important for more fully characterizing the subsurface structure and geology at these locations.

A stable isotope record of Holocene precipitation dynamics in the Baltic region from Lake Nuudsaku, Estonia

A long-term perspective of past hydroclimate variability for the Baltic region provides important context for evaluating the potential impacts of Northern Hemisphere temperature changes and shifting ocean-atmospheric interactions on northern European climate. Lake Nuudsaku is an open-basin lake in south-central Estonia with seasonal overflow fed by active springs and groundwater. Stable isotope analyses of surface water samples from regional lakes, rivers, wetlands, and precipitation indicate that Lake Nuudsaku water δ18O values are consistent with the isotopic composition of inflowing meteoric water. The isotopic composition of fine-grained (<63 μm) endogenic CaCO3 (δ18Ocalcite) samples from Lake Nuudsaku vary between ∼14 to -9‰ (VPBD) over the last ∼9400 years. Based on sensitivity tests, we interpret the δ18Ocalcite variations from Lake Nuudsaku as a record of meteoric inputs; principally reflecting variations in the seasonality of precipitation, with temperature changes as a secondary influence. The δ18Ocalcite record indicates the early Holocene at ∼9400 calendar years before present (cal yr BP) was characterized by a high proportion of cold season (November through April) precipitation and lower temperatures, followed by waning cold season precipitation and higher temperatures until ∼8200 cal yr BP. The middle Holocene (8200–4200 cal yr BP) was likely a period of overall reduced cold season precipitation and higher temperatures, and/or increased warm season (May through October) precipitation. The late Holocene transition at ∼4200 cal yr BP generally marks a shift to lower δ18Ocalcite values that we interpret largely the result of increased cold season precipitation amounts. This was followed by the onset of more variable precipitation amounts and temperatures after ∼3600 cal yr BP, which gave way to less variable conditions during the last ∼2600 years. The Medieval Climate Anomaly (MCA) is marked by δ18Ocalcite values that are near the Holocene and present-day averages. The period spanning the Little Ice Age (LIA) is marked by low δ18Ocalcite values, suggesting temperatures were lower and cold season conditions were wetter than present-day. Proxy evidence from Lake Nuudsaku further indicates that centennial to millennial-scale hydroclimate shifts in the Baltic region were likely associated with changes in North Atlantic ocean-atmospheric circulation dynamics.

Stratigraphy and paleohydrology of delta channel deposits, Jezero crater, Mars

The Jezero crater open-basin lake contains two well-exposed fluvial sedimentary deposits formed early in martian history. Here, we examine the geometry and architecture of the Jezero western delta fluvial stratigraphy using high-resolution orbital images and digital elevation models (DEMs). The goal of this analysis is to reconstruct the evolution of the delta and associated shoreline position. The delta outcrop contains three distinct classes of fluvial stratigraphy that we interpret, from oldest to youngest, as: (1) point bar strata deposited by repeated flood events in meandering channels; (2) inverted channel-filling deposits formed by avulsive distributary channels; and (3) a valley that incises the deposit. We use DEMs to quantify the geometry of the channel deposits and estimate flow depths of ∼7 m for the meandering channels and ∼2 m for the avulsive distributary channels. Using these estimates, we employ a novel approach for assessing paleohydrology of the formative channels in relative terms. This analysis indicates that the shift from meandering to avulsive distributary channels was associated with an approximately four-fold decrease in the water to sediment discharge ratio. We use observations of the fluvial stratigraphy and channel paleohydrology to propose a model for the evolution of the Jezero western delta. The delta stratigraphy records lake level rise and shoreline transgression associated with approximately continuous filling of the basin, followed by outlet breaching, and eventual erosion of the delta. Our results imply a martian surface environment during the period of delta formation that supplied sufficient surface runoff to fill the Jezero basin without major drops in lake level, but also with discrete flooding events at non-orbital (e.g., annual to decadal) timescales.

Holocene climate change in Newfoundland reconstructed using oxygen isotope analysis of lake sediment cores

Carbonate minerals that precipitate from open-basin lakes can provide archives of past variations in the oxygen isotopic composition of precipitation (δ18Oppt). Holocene δ18Oppt records from the circum- North Atlantic region exhibit large fluctuations during times of rapid ice sheet deglaciation, followed by more stable conditions when interglacial boundary conditions were achieved. However, the timing, magnitude, and climatic controls on century to millennial-scale variations in δ18Oppt in northeastern North America are unclear principally because of a dearth of paleo-proxy data. Here we present a lacustrine sediment oxygen isotope (δ18O) record spanning 10,200 to 1200 calendar years before present (calyr BP) from Cheeseman Lake, a small, alkaline, hydrologically open lake basin located in west-central Newfoundland, Canada. Stable isotope data from regional lakes, rivers, and precipitation indicate that Cheeseman Lake water δ18O values are consistent with the isotopic composition of inflowing meteoric water. In light of the open-basin hydrology and relatively short water residence time of the lake, we interpret down-core variations in calcite oxygen isotope (δ18Ocal) values to primarily reflect changes in δ18Oppt and atmospheric temperature, although other factors such as changes in the seasonality of precipitation may be a minor influence. We conducted a series of climate sensitivity simulations with a lake hydrologic and isotope mass balance model to investigate theoretical lake water δ18O responses to climate change. Results from these experiments suggest that Cheeseman Lake δ18O values are primarily controlled by temperature and to a much lesser extent, the seasonality of precipitation. Increasing and more positive δ18Ocal values between 10,200 and 8000calyr BP are interpreted to reflect the waning influence of the Laurentide Ice Sheet on atmospheric circulation, warming temperatures, and rapidly changing surface ocean δ18O from the input of glacial meltwater into the western North Atlantic Ocean. The increasing trend is interrupted by abrupt negative δ18Ocal anomalies at 9700calyr BP, associated with a transition to colder sea surface temperatures (SSTs) in the Labrador Sea and renewed Laurentide Ice Sheet retreat, and at 8500calyr BP that coincides with a well-established cooling event in the circum-North Atlantic region. After 8000calyr BP, δ18Ocal values gradually decrease until 4300calyr BP, which reflects a cooling trend related to declining Boreal summer insolation and lower sea surface temperatures in the western North Atlantic Ocean. δ18Ocal becomes slightly more positive from 4300 to 2500calyr BP and thereafter declines to the most negative values of the late Holocene by 1200calyr BP. The middle to late-Holocene transition at ~4300calyr BP corresponds with a shift to wetter conditions in Newfoundland that is also seen in other paleo-proxy records from the region. The discordance between the Cheeseman Lake δ18Ocal record and declining insolation could in part reflect warmer temperatures or an increase (decrease) in warm (cold) season precipitation. Considering evidence from other paleo-records in Newfoundland, we suggest this transition resulted from a change in synoptic scale atmospheric circulation to a configuration similar to the positive mean state phase of the Pacific-North American pattern. Proxy evidence from Cheeseman Lake therefore supports the idea that a substantial climatic change occurred during the middle to late-Holocene transition (between ~5000 to ~4000calyr BP) in northeastern North America.

Insights into surface runoff on early Mars from paleolake basin morphology and stratigraphy

We present observations on the morphology and stratigraphy of more than 400 paleolake basins on Mars. We show that there are two distinct classes of Martian paleolake basins: (1) paleolakes fed by regionally integrated valley networks (N = 251), and (2) paleolakes fed by isolated inlet valleys not integrated into broader regional drainage systems (N = 174). We conclude that valley network–fed paleolakes primarily formed prior to approximately the Noachian-Hesperian boundary, ca. 3.7 Ga, while isolated inlet valley paleolakes primarily formed later in Martian history. All 174 isolated inlet valley paleolakes are closed-basin lakes; however, there are surprisingly few (31) valley network–fed closed-basin lakes compared to a large number (220) of valley network–fed open-basin lakes. This observation is consistent with declining levels of fluvial activity over time on the Martian surface. Our results imply that during the era of valley network formation, ∼90% of topographic basins breached by an inlet valley had sufficiently high ratios of water influx to losses to fill, overtop, and form an outlet valley. This conclusion provides an important constraint on the balance between surface runoff production and water losses on early Mars that must be satisfied by any model of the early Martian climate and hydrologic cycle.

Hydrothermal alteration and diagenesis of terrestrial lacustrine pillow basalts: Coordination of hyperspectral imaging with laboratory measurements

We investigate an outcrop of ∼187Ma lacustrine pillow basalts of the Talcott Formation exposed in Meriden, Connecticut, USA, focusing on coordinated analyses of one pillow lava to characterize the aqueous history of these basalts in the Hartford Basin. This work uses a suite of multidisciplinary measurements, including hyperspectral imaging, other spectroscopic techniques, and chemical and mineralogical analyses, from the microscopic scale up to the scale of an outcrop.The phases identified in the sample are albite, large iron oxides, and titanite throughout; calcite in vesicles; calcic clinopyroxene, aegirine, and Fe/Mg-bearing clay in the rind; and fine-grained hematite and pyroxenes in the interior. Using imaging spectroscopy, the chemistry and mineralogy results extend to the hand sample and larger outcrop. From all of the analyses, we suggest that the pillow basalts were altered initially after emplacement, either by heated lake water or magmatic fluids, at temperatures of at least 400–600°C, and the calcic clinopyroxenes and aegirine identified in the rind are a preserved record of that alteration. As the hydrothermal system cooled to slightly lower temperatures, clays formed in the rind, and, during this alteration, the sample oxidized to form hematite in the matrix of the interior and Fe3+ in the pyroxenes in the rind. During the waning stages of the hydrothermal system, calcite precipitated in vesicles within the rind. Later, diagenetic processes albitized the sample, with albite replacing plagioclase, lining vesicles, and accreting onto the exterior of the sample. This albitization or Na-metasomatism occurred when the lake within the Hartford Basin evaporated during a drier past climatic era, resulting in Na-rich brines. As Ca-rich plagioclase altered to albite, Ca was released into solution, eventually precipitating as calcite in previously-unfilled vesicles, dominantly in the interior of the pillow. Coordinated analyses of this sample permit identification of the alteration phases and help synthesize the aqueous history of pillow lavas of the Talcott Formation. These results are also relevant to Mars, where volcanically-resurfaced open basin lakes have been found, and this Hartford Basin outcrop may be a valuable analog for any potential volcano–lacustrine interactions. The results can also help to inform the utility and optimization of potentially complementary, synergistic, and uniquely-suited techniques for characterization of hydrothermally-altered terrains.

Glaciation in the Late Noachian Icy Highlands: Ice accumulation, distribution, flow rates, basal melting, and top-down melting rates and patterns

Geological evidence for extensive non-polar ice deposits of Amazonian age indicates that the current cold and dry climate of Mars has persisted for several billion years. The geological record and climate history of the Noachian, the earliest period of Mars history, is less certain, but abundant evidence for fluvial channels (valley networks) and lacustrine environments (open-basin lakes) has been interpreted to represent warm and wet conditions, including rainfall and runoff. Alternatively, recent atmospheric modeling results predict a “cold and icy” Late Noachian Mars in which moderate atmospheric pressure accompanied by a full water cycle produce an atmosphere where temperature declines with elevation following an adiabatic lapse rate, in contrast to the current situation on Mars, where temperature is almost completely determined by latitude. These results are formulated in the Late Noachian Icy Highlands (LNIH) model, in which these cold and icy conditions lead to the preferential deposition of snow and ice at high elevations, such as the southern uplands. What is the fate of this snow and ice and the nature of glaciation in such an environment? What are the prospects of melting of these deposits contributing to the observed fluvial and lacustrine deposits?To address these questions, we report on a glacial flow-modeling analysis using a Mars-adapted ice sheet model with LNIH climate conditions. The total surface/near-surface water inventory is poorly known for the Late Noachian, so we explore the LNIH model in a “supply-limited” scenario for a range of available water abundances and a range of Late Noachian geothermal fluxes. Our results predict that the Late Noachian icy highlands (above an equilibrium line altitude of approximately +1km) were characterized by extensive ice sheets of the order of hundreds of meters thick. Due to extremely cold conditions, the ice-flow velocities in general were very low, less than a fewmm/yr, and the regional ice-flow pattern was disorganized and followed topography, with no radial flow pattern typical of an equilibrium ice sheet. Virtually the entire ice sheet is predicted to be cold-based, and thus the range of wet-based features typically associated with temperate glaciers (e.g., drumlins, eskers, etc.) is not predicted to occur. Wet-based conditions are predicted only locally in the thickest ice (on the floors of the deepest craters), where limited subglacial lakes may have formed.These LNIH regional ice-sheets provide a huge reservoir of potential meltwater as a source for forming the observed fluvial and lacustrine features and deposits. Top-down melting scenarios applied to our LNIH ice sheet model predict that periods of punctuated warming could lead to elevated temperatures sufficient to melt enough snow and ice to readily account for the observed fluvial and lacustrine features and deposits. Our model indicates that such melting should take place preferentially at the margins of the ice sheets, a prediction that can be tested with further analyses.

The climate history of early Mars: insights from the Antarctic McMurdo Dry Valleys hydrologic system

AbstractThe early climate of Mars (Noachian Period, the first ~20% of its history) is thought to differ significantly from that of its more recent history (Amazonian Period, the last ~66%) which is characterized by hyperarid, hypothermal conditions that result in mean annual air temperatures (MAAT) well below 0°C, a global cryosphere, minimal melting on the ground surface, and a horizontally stratified hydrologic system. We explore the nature of the fluvial and lacustrine environments in the Mars-like hyperarid, hypothermal McMurdo Dry Valleys (MDV), where the MAAT is well below 0°C (~ -14 to -30°C) in order to assess whether the Late Noachian geologic record can be explained by a climate characterized by “cold and icy” conditions. We find that the MDV hydrological system and cycle provide important insights into the potential configuration of a “cold and icy” early Mars climate in which MDV-like ephemeral streams and rivers, and both closed-basin and open-basin lakes could form. We review a series of MDV fluvial and lacustrine features to guide investigators in the analysis of the geomorphology of early Mars and we outline a new model for the nature and evolution of a “cold and icy” Late Noachian climate based on these observations. We conclude that a cold and icy Late Noachian Mars with MAAT below freezing, but peak seasonal and peak daily temperatures above 0°C, could plausibly account for the array of Noachian-aged fluvial and lacustrine features observed on Mars. Our assessment also provides insight into the potential effects of punctuated warming on a cold and icy early Mars, in which impact crater formation or massive volcanic eruptions cause temperatures in the melting range for decadal to centennial timescales. We outline a set of outstanding questions and tests concerning the nature and evolution of these features on Mars.

An analysis of open-basin lake deposits on Mars: Evidence for the nature of associated lacustrine deposits and post-lacustrine modification processes

A large number of candidate open-basin lakes (low-lying regions with both inlet valleys and an outlet valley) have been identified and mapped on Mars and are fed by valley network systems that were active near the Noachian–Hesperian boundary. The nature of processes that modified the open-basin lake interiors subsequent to lacustrine activity, and how frequently sedimentary deposits related to lacustrine activity remain exposed, has not been extensively examined. An analysis of 226 open-basin lakes was undertaken to identify evidence for: (1) exposed deposits of possible lacustrine origin and (2) post-lacustrine-activity processes that may have modified or resurfaced open-basin lakes. Spectroscopic data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument were analyzed over identified exposed open-basin lake deposits to assess the mineralogy of these deposits. Particular attention was paid to the possible detection of any component of aqueous alteration minerals (e.g. phyllosilicates, hydrated silica, zeolites) or evaporites (e.g. carbonates, sulfates, chlorides) associated with these exposed deposits. The aim of this paper is to act as a broad survey and cataloguing of the types of lacustrine and post-lacustrine deposits that are present within these 226 paleolake basins. Results of the morphologic classification indicate that 79 open-basin lakes (∼35% of the population) contain exposed deposits of possible lacustrine origin, identified on the basis of fan/delta deposits, layered deposits and/or exposed floor material of apparent lacustrine origin. Additionally, all 226 open-basin lakes examined appear to have been at least partially resurfaced subsequent to their formation by several processes, including volcanism, glacial and periglacial activity, impact cratering and aeolian activity. Results from the analysis of CRISM data show that only 10 (∼29% of the 34 deposits with CRISM coverage) of the exposed open-basin lake deposits contain positively identified aqueous alteration minerals, with one deposit also containing evaporites. The identified hydrated and evaporite minerals include Fe/Mg-smectite, kaolinite, hydrated silica and carbonate, with Fe/Mg-smectite the most commonly identified mineral. These results indicate that hydrated and evaporite minerals are not as commonly associated with lacustrine deposits on Mars as they are on Earth. This suggests in situ alteration and mineral precipitation, a common source of such minerals in terrestrial lakes, was not a major process occurring in these paleo-lacustrine systems, and that the observed minerals are likely to be present as transported material within the lacustrine deposits. The lack of widespread in situ alteration also suggests that either the water chemistry in these paleolake systems was not conducive to aqueous alteration and mineral precipitation, or that the open-basin lake systems were relatively short-lived.

Early Mars climate near the Noachian–Hesperian boundary: Independent evidence for cold conditions from basal melting of the south polar ice sheet (Dorsa Argentea Formation) and implications for valley network formation

Currently, and throughout much of the Amazonian, the mean annual surface temperatures of Mars are so cold that basal melting does not occur in ice sheets and glaciers and they are cold-based. The documented evidence for extensive and well-developed eskers (sediment-filled former sub-glacial meltwater channels) in the south circumpolar Dorsa Argentea Formation is an indication that basal melting and wet-based glaciation occurred at the South Pole near the Noachian–Hesperian boundary. We employ glacial accumulation and ice-flow models to distinguish between basal melting from bottom-up heat sources (elevated geothermal fluxes) and top-down induced basal melting (elevated atmospheric temperatures warming the ice). We show that under mean annual south polar atmospheric temperatures (−100°C) simulated in typical Amazonian climate experiments and typical Noachian–Hesperian geothermal heat fluxes (45–65mW/m2), south polar ice accumulations remain cold-based. In order to produce significant basal melting with these typical geothermal heat fluxes, the mean annual south polar atmospheric temperatures must be raised from today’s temperature at the surface (−100°C) to the range of −50 to −75°C. This mean annual polar surface atmospheric temperature range implies lower latitude mean annual temperatures that are likely to be below the melting point of water, and thus does not favor a “warm and wet” early Mars. Seasonal temperatures at lower latitudes, however, could range above the melting point of water, perhaps explaining the concurrent development of valley networks and open basin lakes in these areas. This treatment provides an independent estimate of the polar (and non-polar) surface temperatures near the Noachian–Hesperian boundary of Mars history and implies a cold and relatively dry Mars climate, similar to the Antarctic Dry Valleys, where seasonal melting forms transient streams and permanent ice-covered lakes in an otherwise hyperarid, hypothermal climate.

An overfilled lacustrine system and progradational delta in Jezero crater, Mars: Implications for Noachian climate

The presence of valley networks and open-basin lakes in the late Noachian is cited as evidence for overland flow of liquid water and thus a climate on early Mars that might have supported precipitation and runoff. Outstanding questions center on the nature of such a climate, its duration and variability, and its cause. Open basin lakes, their interior morphology, and their associated channels provide evidence to address these questions. We synthesize the extensive knowledge of terrestrial open basin lakes, deltaic environments, and fluvial systems to assess these questions with evidence from Jezero crater, a 45km diameter open basin lake and its 15,000km2 catchment area, ∼645-km long drainage network, interior sedimentary facies, and ∼50km long outlet channel system. We document the presence of extensive scroll bars and epsilon cross-bedding, both indicative of meandering distributary channels that are not observed on alluvial fans but are typical of fluvial-deltaic depositional environments. A fluvial-deltaic environment is further supported by the post-formational erosion of the deltaic complex: the present-day “delta front” is actually an erosional escarpment truncating delta plain features with the clay-rich prodelta environment, predicted from facies models to make up the outer third of the complex, having been largely removed by eolian erosion. The extensive development via lateral accretion of scroll bars and epsilon cross-bedding, and the reconstructed sedimentary architecture suggest a stable baselevel, in contrast to an environment of constantly rising and falling baselevel related to variable input and evaporation that would favor incision during lowstands. The development of the outlet channel is interpreted to have provided baselevel control in the Jezero open-basin lake. The maturity of the outlet channel, in contrast to the catastrophically scoured landscapes typical of dam-breach channels, favors a consistent overfilled hydrology for the paleolacustrine environment. Sediment transport modeling studies of other valley network and related deposits on Mars have suggested durations in the decades to centuries range. We review meander migration rates in terrestrial fluvial environments to provide a comparison for considering the temporal stability implied by the evolution of scroll bars; values of 20–40 years are not uncommon for the structures and migration implied by observations in Jezero. Taking sediment accumulation rates from a variety of terrestrial fluvial-lacustrine environments in conjunction with our estimates of the sedimentary basin-fill thickness suggest timescales of the order of 106–107 years, far longer than implied by some sediment transport models, but still a short period of time geologically. The presence of significant residual accommodation space (space available for potential sediment accumulation) in Jezero indicates that sediment transport into the lake terminated before the basin was completely filled. Climate conditions sufficient for sustained overland flow of water in the valley networks are required to fill Jezero crater, to cause its breaching in a non-catastrophic manner, and to form the significant fluvial-deltaic environment of laterally migrating fluvial channels and scroll bars formed with an apparently stable baselevel. The lack of late-stage channel downcutting suggests that the conditions producing overland flow of water into the basin may have ended abruptly. Our estimates of the duration of fluvial activity (of order 106–107 years) suggest longer times than previously suggested (years to centuries) by sediment transport models, but generally relatively short durations from a geologic perspective.

Evidence of early Holocene closed-basin conditions in the Huron-Georgian basins from within the North Bay outlet of the upper Great Lakes

Sub-bottom profiling and coring were undertaken at eight sub-basins along the lower French River and at five small lakes near North Bay, Ontario, to collect stratigraphical and chronological evidence to investigate whether lakes occupying the Huron–Georgian basins during the early- to mid-Holocene became hydrologically closed. All of the coring sites are located within the route of the North Bay outlet that carried outflow from the upper Great Lakes during this period. Sand beds containing organic detritus are present within five cores from Muskrat, Crombie and Deep bays that otherwise are composed of glaciolacustrine rhythmites or fine-grained lacustrine deposits. These sand beds are interpreted to represent intervals when water levels within the sub-basins were lower than present, based on chronology, sediment texture, and macrofossil assemblages. It is inferred that the water surface in the Huron–Georgian basins fell below the level of the Dalles Rapids sill isolating the lower French River sub-basins from the large lake. A core from Depensier Lake, North Bay, contains an organic-rich sand interval within a thicker sand unit barren of organic materials. Macrofossils within this organic-rich interval are interpreted to be evidence of substantially diminished flow through the North Bay outlet channel. Radiocarbon dates of terrestrial macrofossils provide correlation of the sand beds between the French River cores as well as with the organic-rich sand in the Depensier Lake core. The possibility that the sand beds in the French River cores represent flood deposits rather than evidence of hydrologically closed conditions is considered, but rejected, based on the occurrence of multiple peaty layers and the record of shallow water conditions inferred from macrofossils within the upper sand bed of core MUS1, Muskrat Bay, in combination with the evidence of quiescent depositional conditions from similarly aged macrofossils in the core from Depensier Lake. Eight radiocarbon dates from the French River cores are incorporated into an elevation-age plot of paleo-indicators of water levels in the Huron–Georgian basins, using additional data from the literature. This plot and stratigraphic evidence from the Muskrat Bay cores indicates that separate closed-basin intervals occurred between 9.0 and 8.4, and 9.5 and 9.3 ka cal BP (~ 8.1 and 7.6, and ~ 8.5 and 8.3 ka BP). The occurrence of these two closed-basin intervals between 9.6 and 8.4 ka cal BP (~ 8.7 and 7.6 ka BP) implies that run-off derived exclusively from precipitation within the non-glaciated portions of the upper Great Lakes drainage basins was likely insufficient at this time to support an open-basin lake hydrology during the contemporary climate, which was colder and drier than present, without being supplemented from glacial Lake Agassiz overflow and/or Laurentide Ice Sheet meltwater.

The sensitivity of East African rift lakes to climate fluctuations

Sequences of paleo-shorelines and the deposits of rift lakes are used to reconstruct past climate changes in East Africa. These recorders of hydrological changes in the Rift Valley indicate extreme lake-level variations on the order of tens to hundreds of meters during the last 20,000 years. Lake-balance and climate modeling results, on the other hand, suggest relatively moderate changes in the precipitation-evaporation balance during that time interval. What could cause such a disparity? We investigated the physical characteristics and hydrology of lake basins to resolve this difference. Nine closed-basin lakes, Ziway-Shalla, Awassa, Turkana, Suguta, Baringo-Bogoria, Nakuru-Elmenteita, Naivasha, Magadi-Natron, Manyara, and open-basin Lake Victoria in the eastern branch of the East African Rift System (EARS) were used for this study. We created a classification scheme of lake response to climate based on empirical measures of topography (hypsometric integral) and climate (aridity index). With reference to early Holocene lake levels, we found that lakes in the crest of the Ethiopian and Kenyan domes were most sensitive to recording regional climatic shifts. Their hypsometric values fall between 0.23–0.29, in a graben-shaped basin, and their aridity index is above unity (humid). Of the ten lakes, three lakes in the EARS are sensitive lakes: Naivasha (HI = 0.23, AI = 1.20) in the Kenya Rift, Awassa (HI = 0.23, AI = 1.03) and Ziway-Shalla (HI = 0.23, AI = 1.33) in the Main Ethiopian Rift (Main Ethiopian Rift). Two lakes have the graben shape, but lower aridity indices, and thus Lakes Suguta (HI = 0.29, AI = 0.43) and Nakuru-Elmenteita (HI = 0.30, AI = 0.85) are most sensitive to local climate changes. Though relatively shallow and slightly alkaline today, they fluctuated by four to ten times the modern water depth during the last 20,000 years. Five of the study lakes are pan-shaped and experienced lower magnitudes of lake level change during the same time period. Understanding the sensitivity of these lakes is critical in establishing the timing or synchronicity of regional-scale events or trends and predicting future hydrological variations in the wake of global climate changes.

Valley network-fed, open-basin lakes on Mars: Distribution and implications for Noachian surface and subsurface hydrology

A new catalog of 210 open-basin lakes (lakes with outlet valleys) fed by valley networks shows that they are widely distributed in the Noachian uplands of Mars. In order for an outlet valley to form, water must have ponded in the basin to at least the level of the outlet. We use this relationship and the present topography to directly estimate the minimum amount of water necessary to flood these basins in the past. The volumes derived for the largest lakes ( ∼ 3 × 10 4 to ∼ 2 × 10 5 km 3 ) are comparable to the largest lakes and small seas on modern Earth, such as the Caspian Sea, Black Sea, and Lake Baikal. We determine a variety of other morphometric properties of these lakes and their catchments (lake area, mean depth, volume, shoreline development, outlet elevation, and watershed area). Most candidate lakes have volumes proportional to and commensurate with their watershed area, consistent with precipitation as their primary source. However, other lakes have volumes that are anomalously large relative to their watershed areas, implying that groundwater may have been important in their filling. Candidate groundwater-sourced lakes are generally concentrated in the Arabia Terra region but also include the Eridania basin [Irwin, R.P., Howard, A.D., Maxwell, T.A., 2004a. J. Geophys. Res. 109, doi: 10.1029/2004JE002287. E12009; Irwin, R.P., Watters, T.R., Howard, A.D. Zimbelman, J.R., 2004b. J. Geophys. Res. 109, doi: 10.1029/2004JE002248. E09011] and several lakes near the dichotomy boundary. This areal distribution is broadly consistent with where groundwater should have reached the surface as predicted by current models. Both surface runoff and groundwater flow appear to have been important sources for lakes and lake chains, suggesting a vertically integrated hydrological system, the absence of a global cryosphere, and direct communication between the surface and subsurface hydrosphere of early Mars.