Murray Richard W.

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Murray

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Richard W.

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0000-0002-7688-4098

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Trace metal evidence for deglacial ventilation of the abyssal Pacific and Southern Oceans

(American Geophysical Union, 2021-08-17) Pavia, Frank ; Wang, Shouyi ; Middleton, Jennifer L. ; Murray, Richard W. ; Anderson, Robert F.

The deep ocean has long been recognized as the reservoir that stores the carbon dioxide (CO2) removed from the atmosphere during Pleistocene glacial periods. The removal of glacial atmospheric CO2 into the ocean is likely modulated by an increase in the degree of utilization of macronutrients at the sea surface and enhanced storage of respired CO2 in the deep ocean, known as enhanced efficiency of the biological pump. Enhanced biological pump efficiency during glacial periods is most easily documented in the deep ocean using proxies for oxygen concentrations, which are directly linked to respiratory CO2 levels. We document the enhanced storage of respired CO2 during the Last Glacial Maximum (LGM) in the Pacific Southern Ocean and deepest Equatorial Pacific using records of deglacial authigenic manganese, which form as relict peaks during increases in bottom water oxygen (BWO) concentration. These peaks are found at depths and regions where other oxygenation histories have been ambiguous, due to diagenetic alteration of authigenic uranium, another proxy for BWO. Our results require that the entirety of the abyssal Pacific below approximately 1,000 m was enriched in respired CO2 and depleted in oxygen during the LGM. The presence of authigenic Mn enrichment in the deep Equatorial Pacific for each of the last five deglaciations suggests that the storage of respired CO2 in the deep ocean is a ubiquitous feature of late-Pleistocene ice ages.
Deep North Atlantic last glacial maximum salinity reconstruction

(American Geophysical Union, 2021-04-24) Homola, Kira ; Spivack, Arthur J. ; Murray, Richard W. ; Pockalny, Robert ; D'Hondt, Steven ; Robinson, Rebecca

We reconstruct deep water-mass salinities and spatial distributions in the western North Atlantic during the Last Glacial Maximum (LGM, 19–26 ka), a period when atmospheric CO2 was significantly lower than it is today. A reversal in the LGM Atlantic meridional bottom water salinity gradient has been hypothesized for several LGM water-mass reconstructions. Such a reversal has the potential to influence climate, ocean circulation, and atmospheric CO2 by increasing the thermal energy and carbon storage capacity of the deep ocean. To test this hypothesis, we reconstructed LGM bottom water salinity based on sedimentary porewater chloride profiles in a north-south transect of piston cores collected from the deep western North Atlantic. LGM bottom water salinity in the deep western North Atlantic determined by the density-based method is 3.41–3.99 ± 0.15% higher than modern values at these sites. This increase is consistent with: (a) the 3.6% global average salinity change expected from eustatic sea level rise, (b) a northward expansion of southern sourced deep water, (c) shoaling of northern sourced deep water, and (d) a reversal of the Atlantic's north-south deep water salinity gradient during the LGM.
A synthesis of monsoon exploration in the Asian marginal seas

(Copernicus Publications, 2022-10-28) Clift, Peter D ; Betzler, Christian ; Clemens, Steven C ; Christensen, Beth ; Eberli, Gregor P ; France-Lanord, Christian ; Gallagher, Stephen J. ; Holbourn, Ann ; Kuhnt, Wolfgang ; Murray, Richard W ; Rosenthal, Yair ; Tada, Ryuji ; Wan, Shiming

The International Ocean Discovery Program (IODP) conducted a series of expeditions between 2013 and 2016 that were designed to address the development of monsoon climate systems in Asia and Australia. Significant progress was made in recovering Neogene sections spanning the region from the Arabian Sea to the Sea of Japan and southward to western Australia. High recovery by advanced piston corer (APC) has provided a host of semi-continuous sections that have been used to examine monsoonal evolution. Use of the half-length APC was successful in sampling sand-rich sediment in Indian Ocean submarine fans. The records show that humidity and seasonality developed diachronously across the region, although most regions show drying since the middle Miocene and especially since ∼ 4 Ma, likely linked to global cooling. A transition from C3 to C4 vegetation often accompanied the drying but may be more linked to global cooling. Western Australia and possibly southern China diverge from the general trend in becoming wetter during the late Miocene, with the Australian monsoon being more affected by the Indonesian Throughflow, while the Asian monsoon is tied more to the rising Himalaya in South Asia and to the Tibetan Plateau in East Asia. The monsoon shows sensitivity to orbital forcing, with many regions having a weaker summer monsoon during times of northern hemispheric Glaciation. Stronger monsoons are associated with faster continental erosion but not weathering intensity, which either shows no trend or a decreasing strength since the middle Miocene in Asia. Marine productivity proxies and terrestrial chemical weathering, erosion, and vegetation proxiesare often seen to diverge. Future work on the almost unknown Paleogene is needed, as well as the potential of carbonate platforms as archives of paleoceanographic conditions.
The contribution of water radiolysis to marine sedimentary life

(Nature Research, 2021-02-26) Sauvage, Justine ; Flinders, Ashton F. ; Spivack, Arthur J. ; Pockalny, Robert ; Dunlea, Ann G. ; Anderson, Chloe H. ; Smith, David C. ; Murray, Richard W. ; D'Hondt, Steven

Water radiolysis continuously produces H2 and oxidized chemicals in wet sediment and rock. Radiolytic H2 has been identified as the primary electron donor (food) for microorganisms in continental aquifers kilometers below Earth’s surface. Radiolytic products may also be significant for sustaining life in subseafloor sediment and subsurface environments of other planets. However, the extent to which most subsurface ecosystems rely on radiolytic products has been poorly constrained, due to incomplete understanding of radiolytic chemical yields in natural environments. Here we show that all common marine sediment types catalyse radiolytic H2 production, amplifying yields by up to 27X relative to pure water. In electron equivalents, the global rate of radiolytic H2 production in marine sediment appears to be 1-2% of the global organic flux to the seafloor. However, most organic matter is consumed at or near the seafloor, whereas radiolytic H2 is produced at all sediment depths. Comparison of radiolytic H2 consumption rates to organic oxidation rates suggests that water radiolysis is the principal source of biologically accessible energy for microbial communities in marine sediment older than a few million years. Where water permeates similarly catalytic material on other worlds, life may also be sustained by water radiolysis.
Intercomparison of XRF core scanning results from seven labs and approaches to practical calibration

(American Geophysical Union, 2020-09-09) Dunlea, Ann G. ; Murray, Richard W. ; Tada, Ryuji ; Alvarez-Zarikian, Carlos A. ; Anderson, Chloe H. ; Gilli, Adrian ; Giosan, Liviu ; Gorgas, Thomas ; Hennekam, Rick ; Irino, Tomohisa ; Murayama, Masafumi ; Peterson, Larry C. ; Reichart, Gert-Jan ; Seki, Arisa ; Zheng, Hongbo ; Ziegler, Martin

X‐ray fluorescence (XRF) scanning of marine sediment has the potential to yield near‐continuous and high‐resolution records of elemental abundances, which are often interpreted as proxies for paleoceanographic processes over different time scales. However, many other variables also affect scanning XRF measurements and convolute the quantitative calibrations of element abundances and comparisons of data from different labs. Extensive interlab comparisons of XRF scanning results and calibrations are essential to resolve ambiguities and to understand the best way to interpret the data produced. For this study, we sent a set of seven marine sediment sections (1.5 m each) to be scanned by seven XRF facilities around the world to compare the outcomes amidst a myriad of factors influencing the results. Results of raw element counts per second (cps) were different between labs, but element ratios were more comparable. Four of the labs also scanned a set of homogenized sediment pellets with compositions determined by inductively coupled plasma‐optical emission spectrometry (ICP‐OES) and ICP‐mass spectrometry (MS) to convert the raw XRF element cps to concentrations in two ways: a linear calibration and a log‐ratio calibration. Although both calibration curves are well fit, the results show that the log‐ratio calibrated data are significantly more comparable between labs than the linearly calibrated data. Smaller‐scale (higher‐resolution) features are often not reproducible between the different scans and should be interpreted with caution. Along with guidance on practical calibrations, our study recommends best practices to increase the quality of information that can be derived from scanning XRF to benefit the field of paleoceanography.
Relationship of bacterial richness to organic degradation rate and sediment age in subseafloor sediment

(American Society for Microbiology, 2016-06-10) Walsh, Emily A. ; Kirkpatrick, John B. ; Pockalny, Robert ; Sauvage, Justine ; Spivack, Arthur J. ; Murray, Richard W. ; Sogin, Mitchell L. ; D'Hondt, Steven

Subseafloor sediment hosts a large, taxonomically rich and metabolically diverse microbial ecosystem. However, the factors that control microbial diversity in subseafloor sediment have rarely been explored. Here we show that bacterial richness varies with organic degradation rate and sediment age. At three open-ocean sites (in the Bering Sea and equatorial Pacific) and one continental margin site (Indian Ocean), richness decreases exponentially with increasing sediment depth. The rate of decrease in richness with depth varies from site to site. The vertical succession of predominant terminal electron acceptors correlates to abundance-weighted community composition, but does not drive the vertical decrease in richness. Vertical patterns of richness at the open-ocean sites closely match organic degradation rates; both properties are highest near the seafloor and decline together as sediment depth increases. This relationship suggests that (i) total catabolic activity and/or electron donor diversity exerts a primary influence on bacterial richness in marine sediment, and (ii) many bacterial taxa that are poorly adapted for subseafloor sedimentary conditions are degraded in the geologically young sediment where respiration rates are high. Richness consistently takes a few hundred thousand years to decline from near-seafloor values to much lower values in deep anoxic subseafloor sediment, regardless of sedimentation rate, predominant terminal electron acceptor, or oceanographic context.
Marine sedimentary records of chemical weathering evolution in the western Himalaya since 17 Ma

(Geological Society of America, 2021-03-24) Zhou, Peng ; Ireland, Thomas ; Murray, Richard W. ; Clift, Peter D.

The Indus Fan derives sediment from the western Himalaya and Karakoram. Sediment from International Ocean Discovery Program drill sites in the eastern part of the fan coupled with data from an industrial well near the river mouth allow the weathering history of the region since ca. 16 Ma to be reconstructed. Clay minerals, bulk sediment geochemistry, and magnetic susceptibility were used to constrain degrees of chemical alteration. Diffuse reflectance spectroscopy was used to measure the abundance of moisture-sensitive minerals hematite and goethite. Indus Fan sediment is more weathered than Bengal Fan material, probably reflecting slow transport, despite the drier climate, which slows chemical weathering rates. Some chemical weathering proxies, such as K/Si or kaolinite/(illite + chlorite), show no temporal evolution, but illite crystallinity and the chemical index of alteration do have statistically measurable decreases over long time periods. Using these proxies, we suggest that sediment alteration was moderate and then increased from 13 to 11 Ma, remained high until 9 Ma, and then reduced from that time until 6 Ma in the context of reduced physical erosion during a time of increasing aridity as tracked by hematite/goethite values. The poorly defined reducing trend in weathering intensity is not clearly linked to global cooling and at least partly reflects regional climate change. Since 6 Ma, weathering has been weak but variable since a final reduction in alteration state after 3.5 Ma that correlates with the onset of Northern Hemispheric glaciation. Reduced or stable chemical weathering at a time of falling sedimentation rates is not consistent with models for Cenozoic global climate change that invoke greater Himalayan weathering fluxes drawing down atmospheric CO2 but are in accord with the idea of greater surface reactivity to weathering.
Geochemical evidence for initiation of the modern Mekong delta in the southwestern South China Sea after 8 Ma

( 2017-01) Liu, Chang ; Clift, Peter D. ; Murray, Richard W. ; Blusztajn, Jerzy S. ; Ireland, Thomas ; Wan, Shiming ; Ding, Weiwei

Sedimentary records in the southwestern South China Sea reflect the evolving erosion and drainage systems that have operated in Southeast Asia during the Neogene. Analyses of the chemistry and clay mineral composition of sediments from International Ocean Discovery Program (IODP) Site U1433 allow us to examine these processes over the last 17 Ma. Sediment older than 8 Ma was deposited relatively slowly. Sr and Nd isotopes indicate a variable provenance with sequences of less and more altered material accompanied by strong changes in the proportion of smectite. Sediment flux was probably from Indochina, as well as from a more primitive volcanic source, most likely the Palawan ophiolite and/or Luzon. Sediments younger than 8 Ma show a more stable Sr and Nd isotope character, indicating sources close to those seen in the modern Mekong River, although with some influx from smaller rivers draining the Indochina margin especially from 4–8 Ma. Our data are consistent with seismic estimates for an onset to the Mekong in its present location after 8 Ma, following an avulsion from the Gulf of Thailand.
Glacial to Holocene changes in trans-Atlantic Saharan dust transport and dust-climate feedbacks

(American Association for the Advancement of Science, 2016-11-23) Williams, Ross H. ; McGee, David ; Kinsley, Christopher W. ; Ridley, David A. ; Hu, Shineng ; Fedorov, Alexey ; Tal, Irit ; Murray, Richard W. ; deMenocal, Peter B.

Saharan mineral dust exported over the tropical North Atlantic is thought to have significant impacts on regional climate and ecosystems, but limited data exist documenting past changes in long-range dust transport. This data gap limits investigations of the role of Saharan dust in past climate change, in particular during the mid-Holocene, when climate models consistently underestimate the intensification of the West African monsoon documented by paleorecords. We present reconstructions of African dust deposition in sediments from the Bahamas and the tropical North Atlantic spanning the last 23,000 years. Both sites show early and mid-Holocene dust fluxes 40 to 50% lower than recent values and maximum dust fluxes during the deglaciation, demonstrating agreement with records from the northwest African margin. These quantitative estimates of trans-Atlantic dust transport offer important constraints on past changes in dust-related radiative and biogeochemical impacts. Using idealized climate model experiments to investigate the response to reductions in Saharan dust’s radiative forcing over the tropical North Atlantic, we find that small (0.15°C) dust-related increases in regional sea surface temperatures are sufficient to cause significant northward shifts in the Atlantic Intertropical Convergence Zone, increased precipitation in the western Sahel and Sahara, and reductions in easterly and northeasterly winds over dust source regions. Our results suggest that the amplifying feedback of dust on sea surface temperatures and regional climate may be significant and that accurate simulation of dust’s radiative effects is likely essential to improving model representations of past and future precipitation variations in North Africa.
Cenozoic global cooling and increased seawater Mg/Ca via reduced reverse weathering

(Nature Publishing Group, 2017-10-10) Dunlea, Ann G. ; Murray, Richard W. ; Santiago Ramos, Danielle ; Higgins, John A.

Authigenic clay minerals formed on or in the seafloor occur in every type of marine sediment. They are recognized to be a major sink of many elements in the ocean but are difficult to study directly due to dilution by detrital clay minerals. The extremely low dust fluxes and marine sedimentation rates in the South Pacific Gyre (SPG) provide a unique opportunity to examine relatively undiluted authigenic clay. Here, using Mg isotopes and element concentrations combined with multivariate statistical modeling, we fingerprint and quantify the abundance of authigenic clay within SPG sediment. Key reactants include volcanic ash (source of reactive aluminium) and reactive biogenic silica on or shallowly buried within the seafloor. Our results, together with previous studies, suggest that global reorganizations of biogenic silica burial over the Cenozoic reduced marine authigenic clay formation, contributing to the rise in seawater Mg/Ca and decline in atmospheric CO2 over the past 50 million years.
Climatically driven changes in the supply of terrigenous sediment to the East China Sea

(John Wiley & Sons, 2018-08-11) Anderson, Chloe H. ; Murray, Richard W. ; Dunlea, Ann G. ; Giosan, Liviu ; Kinsley, Christopher W. ; McGee, David ; Tada, Ryuji

We examine the paleoceanographic record over the last ∼400 kyr derived from major, trace, and rare earth elements in bulk sediment from two sites in the East China Sea drilled during Integrated Ocean Drilling Program Expedition 346. We use multivariate statistical partitioning techniques (Q‐mode factor analysis, multiple linear regression) to identify and quantify five crustal source components (Upper Continental Crust (UCC), Luochuan Loess, Xiashu Loess, Southern Japanese Islands, Kyushu Volcanics), and model their mass accumulation rates (MARs). UCC (35–79% of terrigenous contribution) and Luochuan Loess (16–55% contribution) are the most abundant end‐members through time, while Xiashu Loess, Southern Japanese Islands, and Kyushu Volcanics (1–22% contribution) are the lowest in abundance when present. Cycles in UCC and Luochuan Loess MARs may indicate continental and loess‐like material transported by major rivers into the Okinawa Trough. Increases in sea level and grain size proxy (e.g., SiO2/Al2O3) are coincident with increased flux of Southern Japanese Islands, indicating localized sediment supply from Japan. Increases in total terrigenous MAR precede minimum relative sea levels by several thousand years and may indicate remobilization of continental shelf material. Changes in the relative contribution of these end‐members are decoupled from total MAR, indicating compositional changes in the sediment are distinct from accumulation rate changes but may be linked to variations in sea level, riverine and eolian fluxes, and shelf‐bypass processes over glacial‐interglacials, complicating accurate monsoon reconstructions from fluvial dominated sediment.
Atribacteria reproducing over millions of years in the Atlantic abyssal subseafloor

(American Society for Microbiology, 2020-10-06) Vuillemin, Aurèle ; Vargas, Sergio ; Coskun, Ömer K. ; Pockalny, Robert ; Murray, Richard W. ; Smith, David C. ; D'Hondt, Steven ; Orsi, William D.

How microbial metabolism is translated into cellular reproduction under energy-limited settings below the seafloor over long timescales is poorly understood. Here, we show that microbial abundance increases an order of magnitude over a 5 million-year-long sequence in anoxic subseafloor clay of the abyssal North Atlantic Ocean. This increase in biomass correlated with an increased number of transcribed protein-encoding genes that included those involved in cytokinesis, demonstrating that active microbial reproduction outpaces cell death in these ancient sediments. Metagenomes, metatranscriptomes, and 16S rRNA gene sequencing all show that the actively reproducing community was dominated by the candidate phylum “Candidatus Atribacteria,” which exhibited patterns of gene expression consistent with fermentative, and potentially acetogenic, metabolism. “Ca. Atribacteria” dominated throughout the 8 million-year-old cored sequence, despite the detection limit for gene expression being reached in 5 million-year-old sediments. The subseafloor reproducing “Ca. Atribacteria” also expressed genes encoding a bacterial microcompartment that has potential to assist in secondary fermentation by recycling aldehydes and, thereby, harness additional power to reduce ferredoxin and NAD+. Expression of genes encoding the Rnf complex for generation of chemiosmotic ATP synthesis were also detected from the subseafloor “Ca. Atribacteria,” as well as the Wood-Ljungdahl pathway that could potentially have an anabolic or catabolic function. The correlation of this metabolism with cytokinesis gene expression and a net increase in biomass over the million-year-old sampled interval indicates that the “Ca. Atribacteria” can perform the necessary catabolic and anabolic functions necessary for cellular reproduction, even under energy limitation in millions-of-years-old anoxic sediments.
High-resolution and high-precision correlation of dark and light layers in the Quaternary hemipelagic sediments of the Japan Sea recovered during IODP Expedition 346

(Springer, 2018-03-26) Tada, Ryuji ; Irino, Tomohisa ; Ikehara, Ken ; Karasuda, Akinori ; Sugisaki, Saiko ; Xuan, Chuang ; Sagawa, Takuya ; Itaki, Takuya ; Kubota, Yoshimi ; Lu, Song ; Seki, Arisa ; Murray, Richard W. ; Alvarez-Zarikian, Carlos A. ; Anderson, William T. ; Bassetti, Maria-Angela ; Brace, Bobbi J. ; Clemens, Steven C. ; da Costa Gurgel, Marcio H. ; Dickens, Gerald R. ; Dunlea, Ann G. ; Gallagher, Stephen J. ; Giosan, Liviu ; Henderson, Andrew C. G. ; Holbourn, Ann E. ; Kinsley, Christopher W. ; Lee, Gwang Soo ; Lee, Kyung Eun ; Lofi, Johanna ; Lopes, Christina I. C. D. ; Saavedra-Pellitero, Mariem ; Peterson, Larry C. ; Singh, Raj K. ; Toucanne, Samuel ; Wan, Shiming ; Zheng, Hongbo ; Ziegler, Martin

The Quaternary hemipelagic sediments of the Japan Sea are characterized by centimeter- to decimeter-scale alternation of dark and light clay to silty clay, which are bio-siliceous and/or bio-calcareous to a various degree. Each of the dark and light layers are considered as deposited synchronously throughout the deeper (> 500 m) part of the sea. However, attempts for correlation and age estimation of individual layers are limited to the upper few tens of meters. In addition, the exact timing of the depositional onset of these dark and light layers and its synchronicity throughout the deeper part of the sea have not been explored previously, although the onset timing was roughly estimated as ~ 1.5 Ma based on the result of Ocean Drilling Program legs 127/128. Consequently, it is not certain exactly when their deposition started, whether deposition of dark and light layers was synchronous and whether they are correlatable also in the earlier part of their depositional history. The Quaternary hemipelagic sediments of the Japan Sea were drilled at seven sites during Integrated Ocean Drilling Program Expedition 346 in 2013. Alternation of dark and light layers was recovered at six sites whose water depths are > ~ 900 m, and continuous composite columns were constructed at each site. Here, we report our effort to correlate individual dark layers and estimate their ages based on a newly constructed age model at Site U1424 using the best available paleomagnetic datum and marker tephras. The age model is further tuned to LR04 δ18O curve using gamma ray attenuation density (GRA) since it reflects diatom contents that are higher during interglacial high-stands. The constructed age model for Site U1424 is projected to other sites using correlation of dark layers to form a high-resolution and high-precision paleo-observatory network that allows to reconstruct changes in material fluxes with high spatio-temporal resolutions.
Quantifying K, U, and Th contents of marine sediments using shipboard natural gamma radiation spectra measured on DV JOIDES Resolution

(John Wiley & Sons, 2017-03-21) De Vleeschouwer, David ; Dunlea, Ann G. ; Auer, Gerald ; Anderson, Chloe H. ; Brumsack, Hans-Jürgen ; de Loach, Aaron ; Gurnis, Michael ; Huh, Youngsook ; Ishiwa, Takeshige ; Jang, Kwangchul ; Kominz, Michelle A. ; März, Christian ; Schnetger, Bernhard ; Murray, Richard W. ; Pälike, Heiko ; Expedition 356 Shipboard Scientists

During International Ocean Discovery Program (IODP) expeditions, shipboard-generated data provide the first insights into the cored sequences. The natural gamma radiation (NGR) of the recovered material, for example, is routinely measured on the ocean drilling research vessel DV JOIDES Resolution. At present, only total NGR counts are readily available as shipboard data, although full NGR spectra (counts as a function of gamma-ray energy level) are produced and archived. These spectra contain unexploited information, as one can estimate the sedimentary contents of potassium (K), thorium (Th), and uranium (U) from the characteristic gamma-ray energies of isotopes in the 40K, 232Th, and 238U radioactive decay series. Dunlea et al. (2013) quantified K, Th, and U contents in sediment from the South Pacific Gyre by integrating counts over specific energy levels of the NGR spectrum. However, the algorithm used in their study is unavailable to the wider scientific community due to commercial proprietary reasons. Here, we present a new MATLAB algorithm for the quantification of NGR spectra that is transparent and accessible to future NGR users. We demonstrate the algorithm's performance by comparing its results to shore-based inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma-emission spectrometry (ICP-ES), and quantitative wavelength-dispersive X-ray fluorescence (XRF) analyses. Samples for these comparisons come from eleven sites (U1341, U1343, U1366-U1369, U1414, U1428-U1430, and U1463) cored in two oceans during five expeditions. In short, our algorithm rapidly produces detailed high-quality information on sediment properties during IODP expeditions at no extra cost.
Archaea dominate oxic subseafloor communities over multimillion-year time scales

(American Association for the Advancement of Science, 2019-06-19) Vuillemin, Aurèle ; Wankel, Scott D. ; Coskun, Ömer K. ; Magritsch, Tobias ; Vargas, Sergio ; Estes, Emily R. ; Spivack, Arthur J. ; Smith, David C. ; Pockalny, Robert ; Murray, Richard W. ; D'Hondt, Steven ; Orsi, William D.

Ammonia-oxidizing archaea (AOA) dominate microbial communities throughout oxic subseafloor sediment deposited over millions of years in the North Atlantic Ocean. Rates of nitrification correlated with the abundance of these dominant AOA populations, whose metabolism is characterized by ammonia oxidation, mixotrophic utilization of organic nitrogen, deamination, and the energetically efficient chemolithoautotrophic hydroxypropionate/hydroxybutyrate carbon fixation cycle. These AOA thus have the potential to couple mixotrophic and chemolithoautotrophic metabolism via mixotrophic deamination of organic nitrogen, followed by oxidation of the regenerated ammonia for additional energy to fuel carbon fixation. This metabolic feature likely reduces energy loss and improves AOA fitness under energy-starved, oxic conditions, thereby allowing them to outcompete other taxa for millions of years.
Zircon U-Pb age constraints on NW Himalayan exhumation from the Laxmi Basin, Arabian Sea

(American Geophysical Union, 2021-12-15) Zhou, Peng ; Stockli, Daniel F. ; Ireland, Thomas ; Murray, Richard W. ; Clift, Peter D.

The Indus Fan, located in the Arabian Sea, contains the bulk of the sediment eroded from the Western Himalaya and Karakoram. Scientific drilling in the Laxmi Basin by the International Ocean Discovery Program recovered a discontinuous erosional record for the Indus River drainage dating back to at least 9.8 Ma, and with a single sample from 15.6 Ma. We dated detrital zircon grains by U-Pb geochronology to reconstruct how erosion patterns changed through time. Long-term increases in detrital zircon U-Pb components of 750–1,200 and 1,500–2,300 Ma record increasing preferential erosion of the Himalaya relative to the Karakoram between 8.3–7.0 and 5.9–5.7 Ma. The average contribution of Karakoram-derived sediment to the Indus Fan fell from 70% of the total at 8.3–7.0 Ma to 35% between 5.9 and 5.7 Ma. An increase in the contribution of 1,500–2,300 Ma zircons starting between 2.5 and 1.6 Ma indicates significant unroofing of the Inner Lesser Himalaya (ILH) by that time. The trend in zircon age spectra is consistent with bulk sediment Nd isotope data. The initial change in spatial erosion patterns at 7.0–5.9 Ma occurred during a time of drying climate in the foreland. The increase in ILH erosion postdated the onset of dry-wet glacial-interglacial cycles suggesting some role for climate control. However, erosion driven by rising topography in response to formation of the ILH thrust duplex, especially during the Pliocene, also played an important role, while the influence of the Nanga Parbat Massif to the total sediment flux was modest.

Murray Richard W.

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