Rapa Nui (Easter Island) Rano Raraku crater lake basin: Geochemical characterization and implications for the Ahu-Moai Period

Rano Raraku, the crater lake constrained by basaltic tuff that served as the primary quarry used to construct the moai statues on Rapa Nui (Easter Island), has experienced fluctuations in lake level over the past centuries. As one of the only freshwater sources on the island, understanding the present and past geochemical characteristics of the lake water is critical to understand if the lake could have been a viable freshwater source for Rapa Nui. At the time of sampling in September 2017, the maximum lake depth was ~1 m. The lake level has substantially declined in the subsequent years, with the lake drying almost completely in January 2018. The lake is currently characterized by highly anoxic conditions, with a predominance of ammonium ions on nitrates, a high concentration of organic carbon in the water-sediment interface and reducing conditions of the lake, as evidenced by Mn/Fe and Cr/V ratios. Our estimates of past salinity inferred from the chloride mass balance indicates that it was unlikely that Rano Raraku provided a viable freshwater source for early Rapa Nui people. The installation of an outlet pipe around 1950 that was active until the late 1970s, as well as grazing of horses on the lake margins appear to have significantly impacted the geochemical conditions of Rano Raraku sediments and lake water in recent decades. Such impacts are distinct from natural environmental changes and highlight the need to consider the sensitivity of the lake geochemistry to human activities

Evaluating a primary carbonate pathway for manganese enrichments in reducing environments

Most manganese (Mn) enrichments in the sedimentary rock record are hosted in carbonate minerals, which are assumed to have formed by diagenetic reduction of precursor Mn-oxides, and are considered diagnostic of strongly oxidizing conditions. Here we explore an alternative model where Mn-carbonates form in redox-stratified water columns linked to calcium carbonate dissolution. In ferruginous Brownie Lake in Minnesota, USA, we document Mn-carbonates as an HCl-extractable phase present in sediment traps and in reducing portions of the water column. Mn-carbonate becomes supersaturated in the Brownie Lake chemocline where dissolved oxygen concentrations fall below 5 μM, and Mn-oxide reduction increases the dissolved Mn concentration. Supersaturation is enhanced when calcite originating from surface waters dissolves in more acidic waters at the chemocline. In the same zone, sulfate reduction and microaerobic methane oxidation add dissolved inorganic carbon (DIC) with negative . These observations demonstrate that sedimentary Mn enrichments may 1) develop from primary carbonate phases, and 2) can occur in environments with dissolved oxygen concentrations 200 μM), and where Mn and Fe are partitioned by S cycling, photoferrotrophy, or microaerophilic Fe-oxidation. A shallow lysocline enhances Mn-carbonate production by providing additional DIC and nucleation sites for crystal growth. This carbonate model for Mn-enrichments is expected to be viable in both euxinic and ferruginous environments, and provides a more nuanced view of the relationships between Mn and carbon cycling, with applications throughout the rock record

Anthropogenic alteration of nutrient supply increases the global freshwater carbon sink

Lakes have a disproportionate effect on the global carbon (C) cycle relative to their area, mediating C transfer from land to atmosphere, and burying organic-C in their sediments. The magnitude and temporal variability of C burial is, however, poorly constrained, and the degree to which humans have influenced lake C cycling through landscape alteration has not been systematically assessed. Here, we report global and biome specific trajectories of lake C sequestration based on 516 lakes and show that some lake C burial rates (i.e., those in tropical forest and grassland biomes) have quadrupled over the last 100 years. Global lake C-sequestration (~0.12 Pg year-1) has increased by ~72 Tg year-1 since 1900, offsetting 20% of annual CO2 freshwater emissions rising to ~30% if reservoirs are included and contributing to the residual continental C sink. Nutrient availability explains ~70% of the observed increase, while rising temperatures have a minimal effect

Scientific drilling of Lake Chalco, Basin of Mexico (MexiDrill)

The primary scientific objective of MexiDrill, the Basin of Mexico Drilling Program, is development of a continuous, high-resolution ∼400 kyr lacustrine record of tropical North American environmental change. The field location, in the densely populated, water-stressed Mexico City region gives this record particular societal relevance. A detailed paleoclimate reconstruction from central Mexico will enhance our understanding of long-term natural climate variability in the North American tropics and its relationship with changes at higher latitudes. The site lies at the northern margin of the Intertropical Convergence Zone (ITCZ), where modern precipitation amounts are influenced by sea surface temperatures in the Pacific and Atlantic basins. During the Last Glacial Maximum (LGM), more winter precipitation at the site is hypothesized to have been a consequence of a southward displacement of the mid-latitude westerlies. It thus represents a key spatial node for understanding large-scale hydrological variability of tropical and subtropical North America and is at an altitude (2240 m a.s.l.), typical of much of western North America. In addition, its sediments contain a rich record of pre-Holocene volcanic history; knowledge of the magnitude and frequency relationships of the area's explosive volcanic eruptions will improve capacity for risk assessment of future activity. Explosive eruption deposits will also be used to provide the backbone of a robust chronology necessary for full exploitation of the paleoclimate record. Here we report initial results from, and outreach activities of, the 2016 coring campaign

Trans-Amazon Drilling Project (TADP): origins and evolution of the forests, climate, and hydrology of the South American tropics

This article presents the scientific rationale for an ambitious ICDP drilling project to continuously sample Late Cretaceous to modern sediment in four different sedimentary basins that transect the equatorial Amazon of Brazil, from the Andean foreland to the Atlantic Ocean. The goals of this project are to document the evolution of plant biodiversity in the Amazon forests and to relate biotic diversification to changes in the physical environment, including climate, tectonism, and the surface landscape. These goals require long sedimentary records from each of the major sedimentary basins across the heart of the Brazilian Amazon, which can only be obtained by drilling because of the scarcity of Cenozoic outcrops. The proposed drilling will provide the first long, nearly continuous regional records of the Cenozoic history of the forests, their plant diversity, and the associated changes in climate and environment. It also will address fundamental questions about landscape evolution, including the history of Andean uplift and erosion as recorded in Andean foreland basins and the development of west-to-east hydrologic continuity between the Andes, the Amazon lowlands, and the equatorial Atlantic. Because many modern rivers of the Amazon basin flow along the major axes of the old sedimentary basins, we plan to locate drill sites on the margin of large rivers and to access the targeted drill sites by navigation along these rivers

Perforación profunda en el lago de Chalco: Reporte técnico

En este artículo se presenta un resumen de las actividades realizadas para la recuperación de la totalidad de la secuencia lacustre del lago de Chalco. Mediante estudios geofísicos se determinó la distribución y espesor de los sedimentos lacustres con base en lo cual se seleccionó el sitio de perforación. Con datos de los espectros H/V de sísmica pasiva se hizo un mapa de isofrecuencias que definieron una región con sedimentos lacustres y material volcánico granulado de hasta 300 m de espesor. El uso de métodos electromagnéticos mostró cambios en la resistividad eléctrica relacionados con variaciones en la composición de la columna sedimentaria; entre 100 – 120 m de profundidad hay un primer aumento en la resistividad asociado al incremento de materiales volcaniclásticos, y entre 330 – 400 m de profundidad un segundo aumento asociado a la presencia de coladas de basalto. Fueron perforados tres pozos con recuperación continua, llegando a profundidades de 420 m en el pozo A, 310 m en el B y 520 en el C. Durante el trabajo de perforación se tomaron muestras para el análisis geomicrobiológicos y de metagenómica. Durante el proceso de perforación se recuperó un total de 1152 m de sedimentos con una profundidad máxima de 520 m. El porcentaje de recuperación de la columna sedimentaria varió entre 88 a 92 % en los tres sondeos. Los resultados del análisis de susceptibilidad magnética en las tres secuencias indica que los primeros 260 m son sedimentos lacustres, entre 260 y 300 m los sedimentos son más gruesos y debajo de los 300 m son predominantemente volcaniclásticos. El análisis de la secuencia sedimentaria del lago de Chalco de los últimos ~300000 años, permitirá documentar y ampliar el conocimiento acerca de la variabilidad climática de la zona, la historia paleoambiental, la historia del cierre de la cuenca, el desarrollo del sistema lacustre y la recurrencia de la actividad volcánica en la cuenca. Además, el estudio de las propiedades físicas de esta secuencia sedimentaria es importante para la modelación de la propagación de ondas sísmicas y de la estructura de la cuenca, así como para mejorar la capacidad de modelación del proceso de subsidencia del terreno que experimenta esta región. This paper presents a short description of the coring operations undertaken to recover the full lacustrine sedimentary sequence from Chalco. Geophysical techniques were used to determine the distribution and thickness of the sediments in order to select the drilling site. Resonance frequencies determined from H/V spectral ratios were used to determine an area where lake sediments reached 300 m thickness. Electromagnetic survey showed two changes in electric resistivity which were related to changes in sediment composition, the first from 100 to 120 m, related to an increase in volcanoclastic sediments and the second from 330 to 400 m related to the presence of a basaltic flows. Three wells were drilled with continuous recovery, reaching depths of 420 m in well A, 310 in B and 520 in C. Samples for geomicrobiological and metagenomics studies were collected during drilling operations. A total of 1152 m of core sediments were recovered reaching a maximum depth of 520 m. Recovery percentages were between 88 and 92 % in the three wells. Magnetic susceptibility analyses in the three sequences show that the first 260 m are mostly lake sediments, between 260 and 300 m sediments are coarser and below 300 m they are mostly volcaniclastic. Analysis of the sedimentary sequence of Lake Chalco that covers the last ~300000 years will allow documenting and extending the knowledge of climate variability in area, the paleoenvironmental history, basin closure history, lacustrian system development and volcanic activity recurrence. Studies of the physical properties of this sequence will be important for seismic propagation and basin structure modeling, and also will improve modeling of the subsidence process that this region experiences