Spatial distribution and flux of terrigenic He dissolved in the sediment pore water of Lake Van (Turkey)

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Abstract

In this study, the largest ever carried out to measure noble gases in the pore water of unconsolidated sediments in lakes, the emission of terrigenic He through the sediment column of Lake Van was successfully mapped on the local scale. The main input of He to the water body occurs at the borders of a deep basin within the lake, which is probably the remains of a collapsed caldera. The 3He/4He ratio identifies the He injected into the sedimentary column of Lake Van as a mixture of He released from a mantle source and radiogenic He of crustal origin (3He/4He2.6-4.1×10-6). During passage through the pore space, terrigenic He seems to be further enriched in radiogenic He that is most likely produced in the sediment column. In fact, two distinct trends in isotopic composition can be distinguished in the He injected from the lake basement into the sediments. One of these characterizes samples from the shallow water, the other characterizes samples from the deep basin. However, both of these trends are related to the same source of terrigenic He. The He fluxes determined seem to be characteristic of each sampling location and might be considered as a proxy for the fluid permeability of the deep sediment column. These new findings provide insight into the process of fluid transport within the sediments and into the process of formation of the lake basin. Moreover, the isotopic signature of the He that emanates into the water column of Lake Van is strongly affected by the mixing conditions prevailing in the overlying water body. This fact misled previous studies to interpret the terrigenic He in Lake Van as being solely of mantle origin (3He/4He10-5).

Introduction

The flux of He from the upper mantle into the oceans (3×109atomsm-2s-1) was estimated by Craig et al. (1975) from 3He surpluses (with respect to the expected atmospheric equilibrium concentrations) determined on the East Pacific Rise (see also Farley et al., 1995, Bianchi et al., 2010, Lan et al., 2010). However, until now only very few direct measurements have been available to assess He release from the continents, which in theory are responsible for most of the terrestrial He release (Mamyrin and Tolstikhin, 1984, Ballentine et al., 2002).

Various studies during the past few decades have tried to quantify continental He emission based on noble-gas and He concentrations (i) in ground waters (Andrews et al., 1985, Torgersen and Clarke, 1985, Martel et al., 1989, Stute et al., 1992, Marty et al., 1993, Pinti and Marty, 1995, Tolstikhin et al., 1996, Pinti et al., 1997, Castro et al., 1998a, Castro et al., 1998b, Castro et al., 2005, Ma et al., 2005); (ii) in crater lakes (Sano et al., 1990, Collier, 1991, Igarashi et al., 1992, Kipfer et al., 1994, Aeschbach-Hertig et al., 1996b, Aeschbach-Hertig et al., 1999, Aeschbach-Hertig et al., 2002) and other lakes (Aeschbach-Hertig et al., 1996a, Hohmann et al., 1998, Peeters et al., 2000, Kipfer et al., 2002); and (iii) in gas wells (Sano et al., 1986). A major constraint in calculating the He fluxes is given by the average renewal rates of lake bottom water and groundwater, and by the residence time of gas in gas wells. These turn out to be notoriously difficult to determine. Moreover, spatial and temporal heterogeneity in the He emission severely limits its determination. As a result, data-based studies involving the direct determination of He emission on a local scale remain scarce.

Although the earth’s global He budget is understood conceptually, a mechanistic understanding of local-scale continental He emission is still lacking. Various observations indicate that He emission is focused on particular geological structures within the crust, and is related to tectonic activity in extension zones, fault zones, calderas, and volcanoes (Mamyrin and Tolstikhin, 1984, Oxburgh et al., 1986, Oxburgh and O’Nions, 1987, Ballentine et al., 2002, Kennedy et al., 1997, Kennedy and van Soest, 2007). These observations show that the release of He from the continents into the atmosphere is highly variable in space and time. However, the fast mixing processes that occur both in air and in water commonly prevent the atmosphere and hydrosphere from recording and tracking the specific points of terrestrial He release.

Pore waters of lacustrine sediments represent an ideal geochemical environment for assessing local He emission, thus, in principle, allowing fluid transport in the uppermost part of the continental crust to be studied. The transport of He within the sediment column can be described by advection and molecular diffusion, where He migrates through the connected pore space of the sediment (Berner, 1975, Imboden, 1975, Strassmann et al., 2005). As a porous medium, lacustrine sediments suppress the strong horizontal and vertical transport (e.g., by turbulent diffusion) which dominates in the overlying bulk water (Lerman et al., 1995, Schwarzenbach et al., 2003). In contrast to the bulk water, pore water in lacustrine sediments are therefore in principle able to record and store the spatial signals of the local He emission. From this conceptual point of view, pore waters in lake sediments are very suitable for analyzing the variability of the He flux on spatial scales smaller than the diameter of the lake being studied.

In this work we present 3He,4He,20Ne and 3H concentration data measured in the pore water of the sediments of Lake Van (Turkey) between 2004 and 2007.

The lake is known to accumulate terrigenic He from a depleted mantle source (Kipfer et al., 1994, Kaden et al., 2010). The release of He into Lake Van seems to be under the control of certain geological and tectonic features, such as fault lines and volcanic activity (Degens et al., 1984, Şarogˇlu et al., 1992).

In order to quantify the terrestrial He emission, we determined the solute transport within the sediments (e.g., the effective diffusivity in the pore space) by analyzing the vertical distribution of 3H in the sediment column (see Strassmann et al., 2005). Based on the vertical concentration gradients of He within the sediment column and the calculated effective diffusivity, local He fluxes were estimated at the geographical positions of the cores within the lake basin.

For the first time, direct measurements of fluxes of terrigenic He covering a region 3600km2 in area are reported. These measurements allow the mapping of local He emission within a lake environment, which we discuss within the context of the surrounding geology and the available seismic profiles. This provides insight into the fluid transport within the sediment pore space and into the formation of the lake basin. The He fluxes determined also provide further evidence for the heterogeneity of He emission from the solid earth.

Section snippets

Study site

Lake Van, the largest soda lake on earth, is located in a tectonically active region of eastern Anatolia (Turkey) at ∼1650 m a.s.l. (Fig. 1). This region is situated in the vicinity of the triple junction of the Eurasian, Afro-Arabian and Persian plates. Many faults, probably also extending from the shores into the lake basin, are described in the Fault Map of Turkey (Şarogˇlu et al., 1992) and reproduced in Fig. 1. The geology of the northern part of the basin is dominated by volcanic rocks

Methods

The sampling and noble-gas analysis methods employed are described in detail by Brennwald et al. (2003). The cores were taken with an Uwitec gravity corer (http://www.uwitec.at). After recovery on board, each core was squeezed and the bulk sediment was transferred into small copper tubes. Each tube was sealed air-tight on both ends with two special clamps to avoid air contamination in the sample.

Noble-gas concentration determinations and isotopic analyses were carried out in the Noble Gas

He concentration profiles

The dissolved He concentrations determined in the pore-water of all cores (see Table 1 for the details related to the sampling stations) are listed in Table 2 and plotted in Fig. 2. From the 140 samples collected, six were affected by major experimental problems (air contamination or partial extrusion of the sample during gas extraction). The data relating to these samples are not reported.

Eight samples from two cores taken in the Erciş basin (cores D1 and D2) appear to be strongly degassed, as

Conclusions and outlook

The observed maximum terrestrial He flux of 1842×108atomsm-2s-1 correlates well with the seismic features (e.g., Litt et al., 2009) along which terrigenic fluids may preferentially migrate in the sediments of Lake Van. Even at such “hot-spots”, however, the flux is from seven to sixteen times lower than the expected continental flux (O’Nions and Oxburgh, 1983). The flux estimates from the He data collected in the water column (1.6×1010atomsm-2s-1 and 23×1010atomsm-2s-1, Section 4.4) indicate

Acknowledgments.

We would like to thank Prof. Dr. Bernard Marty, Dr. Igor N. Tolstikhin, and two anonymous reviewers for their valuable comments on the manuscript. We thank Aysegül Feray Meydan, Ismet Meydan, Münip Kanan, Mete Orhan, Mehmet Sahin, Dr. Mustafa Karabiyikoğlu, Prof. Dr. Sefer örçen and the staff of Yüzüncü Yıl University in Van for their invaluable support during the Lake Van expeditions over the last few years. Thanks are also due to Frank Peeters, Heike Kaden and Andrea Huber of the University

References (69)

  • T.F. Lan et al.

    Evaluating Earth degassing in subduction zones by measuring helium fluxes from the ocean floor

    Earth Planet. Sci. Lett.

    (2010)
  • G. Landmann et al.

    Dating late glacial abrupt climate changes in the 14,570 year long continuous varve record of Lake Van, Turkey

    Palaeogeogr. Palaeocl.

    (1996)
  • T. Litt et al.

    PALEOVAN, International Continental Scientific Drilling Program (ICDP): site survey results and perspectives

    Quaternary Sci. Rev.

    (2009)
  • F. Peeters et al.

    Analysis of deep-water exchange in the Caspian Sea based on environmental tracers

    Deep-Sea Res. I

    (2000)
  • D.L. Pinti et al.

    Noble gases in crude oils from the Paris Basin, France: implications for the origin of fluids and constraints on oil–water–gas interactions

    Geochim. Cosmochim. Acta

    (1995)
  • Y. Sano et al.

    Helium and carbon fluxes in Lake Nyos, Cameroon: constraint on next gas burst

    Earth Planet. Sci. Lett.

    (1990)
  • M. Stiller et al.

    Water transport through Lake Kinneret sediments traced by tritium

    Earth Planet. Sci. Lett.

    (1975)
  • K. Strassmann et al.

    Dissolved noble gases in porewater of lacustrine sediments as palaeolimnological proxies

    Geochim. Cosmochim. Acta

    (2005)
  • I.N. Tolstikhin et al.

    Helium and argon isotopes in rocks, minerals, and related groundwaters: a case study in northern Switzerland

    Geochim. Cosmochim. Acta

    (1996)
  • T. Torgersen et al.

    Helium accumulation in groundwater, I: an evaluation of sources and the continental flux of crustal 4He in the Great Artesian Basin, Australia

    Geochim. Cosmochim. Acta

    (1985)
  • M.P. Unterweger et al.

    Preparation and calibration of the 1978 National Bureau of Standards tritiated-water standards

    Int. J. Appl. Radiat. Isot.

    (1980)
  • W. Aeschbach-Hertig et al.

    Mantle helium in Lake Van and Lake Nemrut, Eastern Turkey

    Terra Abstracts

    (1991)
  • W. Aeschbach-Hertig et al.

    The physical structure and dynamics of a deep, meromictic crater lake (Lac Pavin, France)

    Hydrobiologia

    (2002)
  • W. Aeschbach-Hertig et al.

    Density-driven exchange between the basins of Lake Lucerne (Switzerland) traced with the 3H–3He method

    Limnol. Oceanogr.

    (1996)
  • Ballentine C. J., Burgess R. and Marty B. (2002) Tracing fluid origin, transport and interaction in the crust. In Noble...
  • Ballentine C. J. and Burnard P. G. (2002) Production, release and transport of noble gases in the continental crust. In...
  • Baur H. (1999) A noble-gas mass spectrometer compressor source with two orders of magnitude improvement in sensitivity....
  • R. Bayer et al.

    Performance and blank components of a mass spectrometric system for routine measurement of helium isotopes and tritium by the 3He ingrowth method. Sitzungsberichte der Heidelberger Akademie der Wissenschaften Mathemathisch-naturwissenschaftliche Klasse 5

    (1989)
  • R. Berner

    Diagenetic models of dissolved species in the interstitial waters of compacting sediments

    Am. J. Sci.

    (1975)
  • U. Beyerle et al.

    A mass spectrometric system for the analysis of noble gases and tritium from water samples

    Environ. Sci. Technol.

    (2000)
  • M.S. Brennwald et al.

    Analysis of dissolved noble gases in the pore water of lacustrine sediments

    Limnol. Oceanogr. Methods

    (2003)
  • M.S. Brennwald et al.

    Atmospheric noble gases in lake sediment pore water as proxies for environmental change

    Geophys. Res. Lett.

    (2004)
  • M.C. Castro et al.

    Noble gases as natural tracers of water circulation in the Paris Basin, 2. Calibration of a groundwater flow model using noble gas isotope data

    Water Resour. Res.

    (1998)
  • M.C. Castro et al.

    Noble gases as natural tracers of water circulation in the Paris Basin, 1. Measurements and discussion of their origin and mechanisms of vertical transport in the basin

    Water Resour. Res.

    (1998)
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