Geology topics

L. Anderson

Publications and source records attributed to L. Anderson.

3 recordsLinked to original sources

USGS2016

Water isotope systematics: Improving our palaeoclimate interpretations

The stable isotopes of oxygen and hydrogen, measured in a variety of archives, are widely used proxies in Quaternary Science. Understanding the processes that control δ18O change have long been a focus of research (e.g. Shackleton and Opdyke, 1973; Talbot, 1990 ; Leng, 2006). Both the dynamics of water isotope cycling and the appropriate interpretation of geological water-isotope proxy time series remain subjects of active research and debate. It is clear that achieving a complete understanding of the isotope systematics for any given archive type, and ideally each individual archive, is vital if these palaeo-data are to be used to their full potential, including comparison with climate model experiments of the past. Combining information from modern monitoring and process studies, climate models, and proxy data is crucial for improving our statistical constraints on reconstructions of past climate variability. As climate models increasingly incorporate stable water isotope physics, this common language should aid quantitative comparisons between proxy data and climate model output. Water-isotope palaeoclimate data provide crucial metrics for validating GCMs, whereas GCMs provide a tool for exploring the climate variability dominating signals in the proxy data. Several of the studies in this set of papers highlight how collaborations between palaeoclimate experimentalists and modelers may serve to expand the usefulness of palaeoclimate data for climate prediction in future work. This collection of papers follows the session on Water Isotope Systematics held at the 2013 AGU Fall Meeting in San Francisco. Papers in that session, the breadth of which are represented here, discussed such issues as; understanding sub-GNIP scale (Global Network for Isotopes in Precipitation, (IAEA/WMO, 2006)) variability in isotopes in precipitation from different regions, detailed examination of the transfer of isotope signals from precipitation to geological archives, and the implications of advances in understanding in these areas for the interpretation of palaeo records and proxy data – climate model comparison. Here, we briefly review these areas of research, and discuss challenges for the water isotope community in improving our ability to partition climate vs. auxiliary signals in palaeoclimate data.

Quaternary Science Reviews↗

USGS2012

Initiation of long-term coupled microbiological, geochemical, and hydrological experimentation within the seafloor at North Pond, western flank of the Mid-Atlantic Ridge

Integrated Ocean Drilling Program (IODP) Expedition 336 successfully initiated subseafloor observatory science at a young mid-ocean-ridge flank setting. All of the drilled sites are located in the North Pond region of the Atlantic Ocean (22°45′N, 46°05′W) in 4414–4483 m water depth. This area is known from previous ocean drilling and site survey investigations as a site of particularly vigorous circulation of seawater in permeable 8 Ma basaltic basement underlying a <300 m thick sedimentary pile. Understanding how this seawater circulation affects microbial and geochemical processes in the uppermost basement was the primary science objective of Expedition 336. Basement was cored and wireline-logged in Holes U1382A and U1383C. Upper oceanic crust in Hole U1382A, which is only 50 m west of Deep Sea Drilling Project (DSDP) Hole 395A, recovered 32 m of core between 110 and 210 meters below seafloor (mbsf). Core recovery in basement was 32%, yielding a number of volcanic flow units with distinct geochemical and petrographic characteristics. A unit of sedimentary breccia containing clasts of basalt, gabbroic rocks, and mantle peridotite was found intercalated between two volcanic flow units and was interpreted as a rock slide deposit. From Hole U1383C we recovered 50.3 m of core between 69.5 and 331.5 mbsf (19%). The basalts are aphyric to highly plagioclase-olivine-phyric tholeiites that fall on a liquid line of descent controlled by olivine fractionation. They are fresh to moderately altered, with clay minerals (saponite, nontronite, and celadonite), Fe oxyhydroxide, carbonate, and zeolite as secondary phases replacing glass and olivine to variable extents. In addition to traditional downhole logs, we also used a new logging tool for detecting in situ microbial life in ocean floor boreholes—the Deep Exploration Biosphere Investigative tool (DEBI-t). Sediment thickness was ~90 m at Sites U1382 and U1384 and varied between 38 and 53 m at Site U1383. The sediments are predominantly nannofossil ooze with layers of coarse foraminiferal sand and occasional pebble-size clasts of basalt, serpentinite, gabbroic rocks, and bivalve debris. The bottommost meters of sections cored with the advanced piston corer feature brown clay. Extended core barrel coring at the sediment/ basement interface recovered <1 m of brecciated basalt with micritic limestone. Sediments were intensely sampled for geochemical pore water analyses and microbiological work. In addition, high-resolution measurements of dissolved oxygen concentration were performed on the whole-round sediment cores Major strides in ridge-flank studies have been made with subseafloor borehole observatories (CORKs) because they facilitate combined hydrological, geochemical, and microbiological studies and controlled experimentation in the subseafloor. During Expedition 336, two fully functional observatories were installed in two newly drilled holes (U1382A and U1383C) and an instrument and sampling string were placed in an existing hole (395A). Although the CORK wellhead in Hole 395A broke off and Hole U1383B was abandoned after a bit failure, these holes and installations are intended for future observatory science targets. The CORK observatory in Hole U1382A has a packer seal in the bottom of the casing and monitors/samples a single zone in uppermost oceanic crust extending from 90 to 210 mbsf. Hole U1383C was equipped with a three-level CORK observatory that spans a zone of thin basalt flows with intercalated limestone (~70–146 mbsf), a zone of glassy, thin basaltic flows and hyaloclastites (146–200 mbsf), and a lowermost zone (~200–331.5 mbsf) of more massive pillow flows with occasional hyaloclastites in the upper part.

Integrated Ocean Drilling Program: Preliminary Rep↗

USGS2005

Palaeohydrology of the Southwest Yukon Territory, Canada, based on multiproxy analyses of lake sediment cores from a depth transect

Lake-level variations at Marcella Lake, a small, hydrologically closed lake in the southwestern Yukon Territory, document changes in effective moisture since the early Holocene. Former water levels, driven by regional palaeohydrology, were reconstructed by multiproxy analyses of sediment cores from four sites spanning shallow to deep water. Marcella Lake today is thermally stratified, being protected from wind by its position in a depression. It is alkaline and undergoes bio-induced calcification. Relative accumulations of calcium carbonate and organic matter at the sediment-water interface depend on the location of the depositional site relative to the thermocline. We relate lake-level fluctuations to down-core stratigraphic variations in composition, geochemistry, sedimentary structures and to the occurrence of unconformities in four cores based on observations of modern limnology and sedimentation processes. Twenty-four AMS radiocarbon dates on macrofossils and pollen provide the lake-level chronology. Prior to 10 000 cal. BP water levels were low, but then they rose to 3 to 4 m below modern levels. Between 7500 and 5000 cal. BP water levels were 5 to 6 m below modern but rose by 4000 cal. BP. Between 4000 and 2000 cal. BP they were higher than modern. During the last 2000 years, water levels were either near or 1 to 2 m below modern levels. Marcella Lake water-level fluctuations correspond with previously documented palaeoenvironmental and palaeoclimatic changes and provide new, independent effective moisture information. The improved geochronology and quantitative water-level estimates are a framework for more detailed studies in the southwest Yukon. ?? 2005 Edward Arnold (Publishers) Ltd.

Holocene↗