Variations in weathering intensity across western Greenland based

Variations in weathering intensity across western Greenland based

Variations in weathering intensity across western
Greenland based on radiogenic isotopes

Daniel Z. Fischer, Ellen E. Martin, Jonathan B. Martin, Kelly M. Deuerling, Cheyenne Everhart
Department of Geological Sciences, University of Florida, Gainesville, FL 32611
Introduction

Pb/204Pb
0

20

40

60

80

120

140

14.4

160

18

K
P
0

20

40

60

80

100

120

140

160

B

Pb

B

Pb/

204
206

Conclusions

16

B
15

K

K
P
14

P

K
P

13
0

20

40

60

80

100

120

140

160

Distance from coast (km)

-28

Silt

15.2

Clay
Clay

-32

Silt

Sand

-34

Clay
Silt
Sand

15.0

Clay

Sand

-36

Clay
Silt
Sand

Silt
-38

Sisimiut
Qorlortoq
Kangerlussuaq
Watson River
Bulk Bedload

14.8

Clay/Silt
Sand

Clay
Silt

Clay
Silt

Sand

14.6

Sand

Sand

Bedload Sr isotopes are
relatively consistent across the
transect
Streamwater Sr isotopes display
a clear decrease with distance
from the ice sheet, indicating an
increased extent of weathering
near the coast.

14.4

-42
20

40

60

80

100

120

140

160

18

42

Clay

Clay

40

0

20

40

60

80

100

120

Silt

Silt
Clay

36

Sand

Silt

16

Clay
Silt

Clay
Silt

Clay
Silt

Silt

Sand

15

Sand
Sand

References

Clay

Silt/Sand

Sand

160

Sand

14

Sand

34

0

20

40

60

80

100

Distance from coast (km)

120

140

Bulk bedload data is relatively constant across the glacial foreland
The biotite Pb isotopic values are variable across the glacial foreland. The trend in biotite
isotopes generally matches weathering trends documented by Sr isotopes. This suggests
mineral separates from the geologic record may be useful for tracking the history of
weathering extent in a glacial foreland
Biotite Pb isotopes are less radiogenic in the Watson River proglacial system than at
Qorlortoq. This is surprising because both systems have relatively recently exposed material
and a negative water balance. The isotopic difference in the biotite from these two areas
suggests less chemical weathering in the Qorlortoq watershed than in sediment discharged
from the active glacier in the upper Watson River watershed.
A possible explanation for the difference in weathering extent between the inland deglaciated
and proglacial watersheds may be that actively discharged material in the proglacial system
was stored and weathered subglacially in the glaciers unique, seasonally variable hydrologic
system prior to discharge.
The coastal deglaciated environments exhibit more extensive chemical weathering in the
mineral separates, as expected
The size fractions do not appear to track any of the chosen minerals and thus may not serve
as a suitable record of weathering changes, possibly due to the heterogeneous mixture of
grain sizes left behind in the glacial moraines and till
The isotopic composition of specific mineral separates, such as biotite, has potential to study
weathering in sediments in the geologic past or in locations where water samples are not
available

17

Clay
38

140

13

-40

0

Pb/204Pb extracted from moraine soils follows a similar weathering trend to Sr isotopes (not
shown), but this signal is complicated by anthropogenic contamination in waters
In general, the most radiogenic ratios of Pb and Nd are found in clay (Pb) and clay/silt (Nd)
and the least radiogenic ratios are in the sand fraction
The coarse fraction (sand) dominates the size fractions by weight percent and isotopic
composition of the bulk samples
Bulk isotopic values outside of the range of values for all of the size fractions suggest inherent
heterogeneity in the bulk sample material.
Silt and clay fractions dominate the bulk bedload Nd signal
The feldspars dictate the bulk bedload Pb signal due to their common nature
The most radiogenic biotite Pb isotopes occur in Qorlortoq, an inland deglaciated
environment, suggesting incompletely developed weathering in this environment that may
relate to discharge from channelized drainage systems at the GrIS base.
208

17

Bedload Size Fractions

Fig. 1- Location Map of Watersheds:

Proglacial watershed: Watson River (yellow star)

Coastal deglaciated watersheds: Sisimiut (light blue circle),
Nerumaq (dark blue square)

Inland deglaciated watersheds: Qorlortoq (orange triangle), and
Lake Helen (red square)

Moraine ages (in white).

Fig. 2 87Sr/86Sr of stream waters
(colored symbols) and bulk bedload
(horizontal black lines) across the
transect (Deuerling et al., in prep)

100

K
P

Fig. 3 Preliminary lead data of common
minerals (orthoclase, plagioclase) and biotite
found in the watersheds. B - biotite, K orthoclase, P - plagioclase. Biotite, with
elevated Rb content, is important to Sr
isotopic values in the watersheds and their
associated weathering patterns. All minerals
were handpicked from the sand fractions (63
m to 2mm) of each site. The most m to 2mm) of each site. The most
radiogenic Pb isotope values were
consistently derived from biotite.

50 km

Sr Isotope Weathering Trends

Distance from coast (km)

Watson
River

~7.3 ky ~6.8 ky

207

P

-30

~9.2 ky ~8.5 ky

WR Minerals

P

Pb/

~9.9 ky

34

Lake Helen

Qorlortoq

K

Bulk Bedload

Pb

Sisimiut
Nerumaq

K
P

Pb/204Pb

+ water
balance

Qor. Minerals

207

Field Area: Western Greenland

K

B

14.8

B

Sis. Minerals

B
K
P

204

36

206

B

38

Nd

15.0

Pb/204Pb Bedload

B

14.6

208

B

15.2

40

Pb/204Pb Bedload

Results

208

Physical weathering of continental ice sheets enhances chemical weathering, which impacts
fluxes of radiogenic isotopes and nutrients to the oceans and atmospheric fluxes of carbon
The western margin of Greenland, which represents a region of retreat of a modern
continental ice sheet, provides a good location to assess spatiotemporal variations of glacial
weathering environments. These environments include: 1) proglacial watersheds, where the
water is sourced directly from the ice sheet, and 2) deglaciated watersheds, which are
hydrologically separated from the ice sheet and only drain precipitation and permafrost melt.
Chemical weathering of fresh material preferentially releases radiogenic Sr and Pb isotopes
due to preferential weathering of minerals enriched in radiogenic isotopes and leaching of
radiogenic Sr and Pb from radiation-damaged crystals (Blum and Erel 2003). This creates an
offset between the isotopic composition of weathered solution and weathered rock (bedload).
The bedload-water offset decreases as extent of weathering increases, allowing a measure
of the extent of weathering
Scribner et al. (2015) documented an increase in the extent of weathering across western
Greenland from the ice sheet toward the coast that coincided with increasing moraine ages
and a transition from a negative to positive water balance (see Fig. 1).
As the Greenland Ice Sheet (GrIS) retreats in response to global warming, the proportion of
deglaciated to proglacial watersheds will increase and the water balance will shift inland.
These changes in response to ice sheet retreat could account for a documented rapid
increase in seawater-derived Pb isotopes during Termination 1, following the last glacial max
Here we show that differences in isotopic composition of minerals are similar to the variation
in water compositions, suggesting that solid phases may record the geologic history of ice
sheet retreat

Bedload Mineral Separates

160

0

20

40

60

80

100

120

140

160

Blum and Erel, 1995, Nature, 373, 415-418.
Hindshaw et al., 2014, GCA, 145, 50-71.
Blum and Erel, 2003, Tr. on Geochem., 5, 365392.
Garon et al., 2014, Chem. Geol., 364, 42-55.
Bhatia et al., 2011, Jour. of Glac., 57, 929-941

Harlavan and Erel, 2002, GCA, 66, 837-848.
Scribner et al., 2015, GCA, 170, 157-172.
Kurzweil et al., 2010, EPSL, 299, 458-465.
Crocket et al., 2012, Quat. Sci. Rev., 38, 89-99.
Harlavan, Erel, Blum, 1998, GCA, 62, 33-46

Distance from coast (km)

Fig. 4 Lead and neodymium isotopic analyses of separated sand, silt, and clay fractions of
bedload from one coastal deglaciated, two inland deglaciated, and the Watson River proglacial
watersheds. Different grain size fractions generally produce unique isotopic ratios. The <2 mm fraction of bedload samples was separated into sand (63 m to 2mm) of each site. The most m to 2mm), silt (2-63 m to 2mm) of each site. The most m), and clay (<2 m to 2mm) of each site. The most m) through sieving and centrifugation. The sand fraction dominated all bedload samples by weight, ranging from 80 wt% in Kangerlussuaq to >99 wt% in Sisimiut.

Acknowledgements
We thank George Kamenov for analytical support. Funding for expeditions and analyses provided
by the NSF Grant PLR-1203773 to JBM and EEM.

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