1. Pb and Sr Isotopes as Tracers of Weathering in Glacial Systems
H41B-1231
Pb and Sr Isotopes as Tracers of Weathering in Glacial Systems
Ellen E. Martin Kelly M. Deuerling, Cecilia Scribner,
Jonathan B. Martin, and Kaylin Clements
Pb Isotopic Data
Introduction
Studies of soil chronosequences from mountain glaciers demonstrate that early stages of weathering preferentially release radiogenic Pb from accessary minerals such as allanite and apatite1,2,3 and radiogenic Sr predominantly from biotite3.
This incongruent weathering produces weathering solutions that are highly enriched in radiogenic Pb and Sr. As weathering continues and sources of radiogenic isotopes are depleted, the isotopic compositions of weathering fluxes approach the bulk rock composition2,4.
Pb in the weathering solutions is ultimately transported to the ocean where it has a short residence time and precipitates out in FeMn oxyhydroxides, preserving a record of local continental weathering. Sr has a much longer residence time in the ocean and mixes with seawater.
Rapid increases in seawater Pb isotope ratios recorded in the North Atlantic over the last deglacial are generally attributed to incongruent weathering of fresh glacial rock flour exposed by retreat of the Laurentide Ice Sheet5,6,7.
This theory is largely untested because little work has been done in modern river systems, especially high latitude rivers, to evaluate the response of radiogenic Pb and Sr isotopes to changing glacial conditions.
In this study we compared Pb and Sr isotopes from sediment and water samples collected along a proglacial river (the Watson River) and several lakes in deglaciated terrains near Kangerlussuaq, Greenland to evaluate the record and extent of weathering in a range of glacial environments. A B
Leachates of moraine soils illustrate:
the greatest offset between the soil and extractable Pb isotopes in the youngest moraine material, consistent with Harlavan et al.1
a decrease in the offset with age of the moraine and exposure time
the labile, highly radiogenic Pb comprises a small fraction of the total Pb, as evidenced by the minimal offset between the bulk and leached soils
206Pb/204Pb for soils, 1N HCl leaches and leached soils for moraine soils. B C A
Sr isotope ratios for bedload (red squares), bedload leached in HH to remove FeMn oxyhydroxides (purple diamonds), and filtered water (dark blue triangles), plus Sr2+ concentrations in waters (inverted light blue triangles) from (A) the Watson River plotted against distance from the ice sheet, and (B) lakes in the deglaciated area..
Sr Isotopic Data
Sr isotopes and Sr2+ concentrations increase downstream in the Watson River, suggesting continuous incongruent weathering downstream.
Sr isotopic data is currently limited and discontinuous in the deglaciated, but the data again indicate incongruent weather, with perhaps the most incongruent weathering in the youngest sediments. High Sr2+ at DG corresponds to high Specific Conductivity as a function of evaporation.
206Pb/204Pb, B) 207Pb/204Pb, C) 208Pb/204Pb of bedload (red squares), bedload leached in hydroxylamine hydrochloride to remove FeMn oxyhydroxides (purple diamonds), and filtered water (blue triangles) from the Watson River plotted against distance from the ice sheet. Error bars are smaller than symbols. Isotopic ratios of anthropogenic Pb10, 11 highlighted by gray shading. Pb concentrations in the water (inverted light blue triangles) are illustrated in (A).
Implications:
Pb isotopes from proglacial river waters record incongruent weathering with enhanced weathering of thorogenic minerals.
Close to the ice sheet Pb isotopes in the proglacial river are contaminated by anthropogenic Pb, which is probably sourced from glacial meltwater.
A leaching experiment with moraine soils suggests fresher material yields more radiogenic Pb isotopes. This implies that waters in deglaciated areas in contact with the sediments for a longer time should record less incongruent weathering; however, Pb isotopes in this environment are extensively contaminated by anthropogenic Pb.
Sr isotopes and concentrations record a clear incongruent weathering signal in both the proglacial river and deglaciated lakes, with an indication of increased weathering downstream. Additional data from deglaciated areas are required to better understand weathering processes in that environment.
Although seawater Sr isotopes are homogenized and record a global signature, it appears that Sr isotopes can serve as a proxy for weathering processes on land and those results can inform the interpretation of geologic time scale, local continental weathering processes recorded by Pb isotopes in marine sediments.
Watson River:
Waters consistently have more radiogenic Pb isotopes than bedload, yet the minimal offset between bedload and leached bedload suggests the leachable radiogenic Pb in the water is a small fraction of the total Pb.
Bedload Pb isotopes decrease slightly downstream, and there is a poorly defined decrease downstream for waters.
Study Area and Methods
Sediment and water samples were collected during a June/July 2012 field season from the Archean Block of western Greenland, which is dominated by Archean gneisses metamorphosed to amphibolite facies.
Water samples were filtered with a 0.45 mm filter and stored in acid washed containers.
Sediment samples were collected with plastic scoops and stored in whirlpack bags.
Pb and Sr were isolated using standard column chemistry at UF. All Pb isotopes and Sr isotopes for solids were analyzed on a Nu Plasma MC-ICPMS. Sr isotopes for waters were analyzed on a VG Sector 54 TIMS. A B
206Pb/204Pb of bedload, leached bedload, and filtered water plus Pb concentrations in the water from lakes in the deglaciated area. Isotopic ratios of anthropogenic Pb10, 11 highlighted by gray shading.
Subglacial
Deglaciated
Proglacial
High physical
weathering
Low chemical
Moderate physical
Moderate chemical
Low physical
weatherin
High chemical
Weathering
Samples were collected from proglacial and deglacial areas characterized by different extents of physical and chemical weathering. Proglacial waters are sourced from subglacial flow and surface runoff during the melt season. Deglacial waters are sourced from local precipitation. There is a negative water budget in the region with precipitation of ~150 mm/yr8. Figure modified from Anderson9.
Deglaciated Lakes:
Pb isotopes of waters are consistently more radiogenic that sediment and have values similar to anthropogenic Pb.
There are no discernible trends in Pb isotopes of the sediment or Pb2+ concentrations in the water.
High physical
weathering
Low chemical
Moderate physical
weathering
Moderate chemical
Weathering
Low physical
weathering
High chemical
Weathering
A) 206Pb/204Pb vs. 207Pb/204Pb, and B) 206Pb/204Pb vs. 208Pb/204Pb for bedload, waters, and HH leachates of bedload in the proglacial Watson River. Gray fields highlight ranges of anthropogenic Pb found in Greenland10,11.
Pb Sources:
206Pb/204Pb vs. 207Pb/204Pb plot illustrates river waters are offset from bedload trend in direction of leachable lead, suggesting incongruent weathering, although the two samples closest to the ice sheet appear to be contaminated by anthropogenic Pb.
208Pb/204Pb in river waters is similar to leachable Pb, suggesting a strong weathering contribution from thorogenic minerals
Lake waters in deglaciated areas are dominated by anthropogenic Pb. A B
Acknowledgements:
We thank George Kamenov for his support with analyses on the Nu MC-ICP and his help with interpretations and Mike Davlantes for map images. This work was supported by National Geographic (JBM), a UF Faculty Enhancement Opportunity (EEM) and NSF ARC (JBM and EEM). We thank CH2M Hill and KISS research facilities for logistical support in the field work.
References:
1 Harlavan, Erel, and Blum, 1998, GCA, 62,
2 Harlavan, Erel, 2002, GCA, 66,
3 von Blanckenburg and Nägler, 2001, Paleoceanography, 16,
4 Blum and Erel, 1997, GCA, 61,
5 Foster and Vance, 2006, Nature 444,
6 Kurzweil, Gutjahr, Vance and Keigwin, 2010, EPSL, 3-4,
7 Crocket, Vance, Foster, Richars and Tranter, Quat. Sci. Rev., 38,
8 Anderson, 2007, Annu. Rev. Earth Plant. Sci., 35,
9 Helweg, 2004, IAHS Pub. 290,
10 Bindler, Renberg, Anderson, Appleby, Emteryd and Boyle, 2001,
Atm. Environ., 35,
11 Gobeil, Macdonald, Smith and Beaudin, 2001, Science,
A road from Kangerlussuaq to the Russell Glacier provided access to a transect of 6 samples that were collected along the proglacial Watson River (yellow symbols). Seven lake samples (orange symbols) and 6 moraine soils (blue symbols) were collected from the deglacial area.
A) 206Pb/204Pb vs. 207Pb/204Pb, and B) 206Pb/204Pb vs. 208Pb/204Pb for sediment, waters, and HH leachates of sediment from lakes in the deglaciated area. Gray fields highlight ranges of anthropogenic Pb found in Greenland10,11.