PUBLICATIONS
Geophysical Research Letters
RESEARCH LETTER
10.1002/2016GL068513
Key Points:• Aquatic plant leaf wax hydrogenisotopes at this study site reﬂect
precipitation seasonality• Winter snowfall on western Greenland
was highest 6 to 4 ka, coincident withsea ice minima• Increased winter snowfall may explain
minimal Holocene Greenland Ice
Sheet retreat

Supporting Information:• Supporting Information S1• Table S1• Table S2• Table S3• Table S4
Correspondence to:E. K. Thomas,
ekthomas@buffalo.edu

Citation:
Thomas, E. K., J. P. Briner, J. J. RyanHenry, and Y. Huang (2016), A major
increase in winter snowfall during the
middle Holocene on western Greenland
caused by reduced sea ice in Bafﬁn Bay
and the Labrador Sea, Geophys. Res.
Lett., 43, 5302–5308, doi:10.1002/
2016GL068513.
Received 3 MAR 2016
Accepted 21 APR 2016
Accepted article online 25 APR 2016
Published online 23 MAY 2016

A major increase in winter snowfall during the middle
Holocene on western Greenland caused by reduced
sea ice in Bafﬁn Bay and the Labrador Sea
Elizabeth K. Thomas1,2, Jason P. Briner2, John J. Ryan-Henry3,4,5, and Yongsong Huang3,61

Department of Geosciences, University of Massachusetts Amherst, Amherst, Massachusetts, USA, 2Department of Geology,
University at Buffalo, Buffalo, New York, USA, 3Department of Earth, Environment and Planetary Sciences, Brown University,
Providence, Rhode Island, USA, 4Department of Marine Affairs, University of Rhode Island, Kingston, Rhode Island, USA,
5
Roger Williams University School of Law, Bristol, Rhode Island, USA, 6Institute of Earth Environment, Chinese Academy of
Sciences, Xi’an, China

Abstract Precipitation is predicted to increase in the Arctic as temperature increases and sea ice retreats.
Yet the mechanisms controlling precipitation in the Arctic are poorly understood and quantiﬁed only by
the short, sparse instrumental record. We use hydrogen isotope ratios (δ2H) of lipid biomarkers in lake
sediments from western Greenland to reconstruct precipitation seasonality and summer temperature during
the past 8 kyr. Aquatic biomarker δ2H was 100‰ more negative from 6 to 4 ka than during the early and late
Holocene, which we interpret to reﬂect increased winter snowfall. The middle Holocene also had highsummer air temperature, decreased early winter sea ice in Bafﬁn Bay and the Labrador Sea, and a strong,warm West Greenland Current. These results corroborate model predictions of winter snowfall increases
caused by sea ice retreat and furthermore suggest that warm currents advecting more heat into the polar
seas may enhance Arctic evaporation and snowfall.

1. Introduction
Arctic precipitation is predicted to increase as climate warms, due to increasing meridional vapor transport andto increasing local evaporation as sea ice retreats [Bintanja and Selten, 2014; Kopec et al., 2016]. Increasing precipitation will be particularly inﬂuential over the Greenland Ice Sheet (GrIS). Greenland’s proximity to abundant
North Atlantic moisture sources is likely one of the main reasons that the GrIS persisted during interglacial periods, when other Northern Hemisphere ice sheets succumbed to rising temperature. Greenland’s ice mass loss,
which is important for sea level rise, is predicted to increase in the coming century, but the rate of ice mass loss
is poorly constrained [Church et al., 2013]. Paired climate and ice sheet records from previous warm periods canelucidate the factors inﬂuencing GrIS mass balance on time scales longer than the observational record [Briner
et al., 2016]. During the middle Holocene, temperature on Greenland was ~ 2°C higher than present [Cuffey andClow, 1997; Axford et al., 2013]. Yet glacial geologic reconstructions and models indicate that after an early
Holocene retreat, the GrIS remained relatively stable from 7 ka to present [Lecavalier et al., 2014; Young andBriner, 2015]. This evidence suggests that some mechanisms, perhaps increasing accumulation, at least partially
offset ice mass losses caused by rising temperature.
The seasonality of increasing precipitation in a warming climate is also particularly important for the GrIS.
Increased winter accumulation can partially offset mass losses due to rising summer temperature. Increased summer rainfall, however, can enhance ablation via latent heat release when rain refreezes, and transient increases inlate-summer ablation can accelerate ice mass loss [Doyle et al., 2015]. Arctic precipitation is difﬁcult to monitor,
but existing observations and reanalysis data suggest that reduced sea ice is correlated to increased precipitation[Sodemann et al., 2008; Kopec et al., 2016]. Models predict that 21st century precipitation increases will be greatest
in fall and winter, caused by a loss of sea ice during early winter [Bintanja and Selten, 2014]. Evaporation is greatest
in the Arctic during early winter and over open water, so a loss of early winter sea ice can cause large increases in
evaporation and concomitant increases in early winter snowfall [Bintanja and Selten, 2014].

©2016. American Geophysical Union.
All Rights Reserved.

THOMAS ET AL.

Hydroclimate and temperature reconstructions during previous warm periods provide climatic context for GrIS
changes. Current continuous reconstructions of Holocene hydroclimate are limited to ice cores, from which it is
difﬁcult to reconstruct precipitation seasonality [Cuffey and Clow, 1997; Rasmussen et al., 2013]. We use hydrogen isotope ratios (δ2H) of lipid biomarkers to reconstruct Holocene precipitation seasonality and temperature

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Geophysical Research Letters10.1002/2016GL068513

on western Greenland near the GrIS
margin, thus better constraining
Arctic precipitation responses tochanging climate.

2. Methods and Approach

13.5765

Figure 1. (a) Map of Greenland showing location of Lake N3 (black and graybulls eye), modeled extents of the GrIS in middle Holocene (bold outline) andmodern (ﬁne outline) [Lecavalier et al., 2014], regional ocean currents(WGC = West Greenland Current, IC = Irminger Current, and EGC = East
Greenland Current), GNIP stations (stars: Thule (N Greenland); Kangilinnguit(S Greenland)) [IAEA/WMO, 2011], paleoceanographic reconstructions (black
and white bulls eyes) [Ledu et al., 2010; de Vernal et al., 2013; Perner et al.,2013; Ouellet-Bernier et al., 2014], and sites mentioned in text
(S = Sukkertoppen Ice Cap, i = Ilulissat and North Lake, G = GISP2, and2
N = NEEM). (b) Monthly temperature, precipitation, and calculated δ Hprecipin Disko Bugt [Bowen et al., 2005; NCDC NOAA, 2012; Bowen, 2016].
Temperature and precipitation calculated for years with complete data, A.D.
1873 to 2013. 1 sigma standard deviations of long-term mean observations
shown for temperature (vertical gray lines, n = 113) and precipitation (whitelines inside of black bars, n = 117).

Lake N3 (informal name, 68.636°N,
50.980°W, 59 m above sea level) is asmall lake (0.09 km2, maximum
depth = 16 m) on the Nuuk Peninsula,
southeastern Disko Bugt, western
Greenland (Figure 1 and Figure S1 in
the supporting information). Lake N3
is through-ﬂowing during the ice-freeseason (May to September) and sincedeglaciation circa 8 ka has notreceived melt water from theGreenland Ice Sheet [Young et al.,2013]. Vegetation in the Lake N3
catchment today is dwarf shrub heath,dominated by Salix sp. and Betula sp.
[Bennike, 2000]. Our paleoclimate
records are generated from a surface
core (10N3-SC) and along core
(10N3-2A) collected from deep basinsin Lake N3 in August 2010 (Figure S1
in the supporting information). An
age-depth model for 10N3-SC wasdeveloped by applying a constant rateof supply model to the unsupported
210
Pb inventory (Figure S2 and Table
S1 in the supporting information). An
age-depth model for 10N3-2A wasdetermined using a smooth spline(smoothing level = 0.3) interpolation
between eight 14C-dated Warnstorﬁasp. (aquatic plant) macrofossils(Figure S2 and Table S2 in the supporting information). Bulk andcompound-speciﬁc geochemical analyses followed previously publishedmethods [Thomas et al., 2011, 2012](the supporting information).

Today, there are large precipitationhydrogen isotope (δ2Hprecip) gradients on western Greenland, corresponding to large climatic gradients (Figure S3 in the supporting information). Kangilinnguit (southern
Greenland) has a maritime climate, with constant δ2Hprecip values year round, indicative of a strong inﬂuenceof local moisture sources [IAEA/WMO, 2011; NCDC NOAA, 2012]. Thule (northern Greenland) has a high-Arctic
continental climate, with large-amplitude seasonal changes in temperature and δ2Hprecip [IAEA/WMO, 2011;
NCDC NOAA, 2012]. Seasonal temperature and δ2Hprecip in the Disko Bugt region have similar patterns as at
Thule but with muted amplitudes [Bowen et al., 2005; NCDC NOAA, 2012; Bowen, 2016]. The northward decreasein annual average δ2Hprecip ( 94‰ in southern Greenland, 132‰ in Disko Bugt, and 183‰ in northern
THOMAS ET AL.

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Geophysical Research Letters10.1002/2016GL068513

Greenland) but increase in seasonalδ2Hprecip amplitudes along western
Greenland is likely a result of a northward increase in distillation during
vapor transport [Jouzel et al., 1997].
Distillation and temperature arenegatively correlated, and as a result,the modern temperature-δ2Hprecip
relationship increases northward
along western Greenland, from
0.1‰°C 1 at Kangilinnguit to
3.4‰°C 1 at Thule [IAEA/WMO,
2011] (Figure S3 in the supporting
information). Temperature-δ2Hprecip
relationships likely changed throughtime, as an increase in sea ice cover
would increase transport distanceand cause more distillation duringtransport [Jouzel et al., 1997; Faber
et al., 2015]. Thus, even if temperatureis the main mechanism inﬂuencing
past δ2Hprecip, records of precipitation isotopes in this region should
be used to infer only qualitativechanges in temperature, unless independent constraints can be placedon the temperature-δ2Hprecip paleorelationship.

2

Figure 2. (a) Lake N3 δ Haq (y axis inverted, gray shading = 95% age model
uncertainty, vertical black lines = analytical uncertainty). (b) Months withgreater than 50% sea ice cover inferred from dinocysts in the Labrador Sea(ﬁne line) and Disko Bugt (bold line), and abundance of sea ice-associateddiatom Fragilariopsis cylindrus Disko Bugt (dashed line) [de Vernal et al., 2013;
Krawczyk et al., 2013; Ouellet-Bernier et al., 2014]. Data smoothing is fromoriginal publications. (c) Percent ﬁne sand in Disko Bugt sediments, indicative of West Greenland Current strength [Perner et al., 2013]. (d)Concentration of Atlantic calcareous benthic foraminifera from two cores,
indicative of warm West Greenland Current [Perner et al., 2013]. (e) Months
with sea ice cover > 50% inferred from dinocysts in northern Bafﬁn Bay [Leduet al., 2010; de Vernal et al., 2013]. Data smoothing is from original publications. (f) Accumulation at NEEM ice core site [Rasmussen et al., 2013].

We generate Holocene records ofprecipitation seasonality and summer
temperature changes using leaf wax
δ2H. δ2H of leaf waxes from surface
sediments in temperate lakes and in
Arctic lakes that experience minimal
evaporative enrichment is correlatedwith precipitation and lake water
δ2H, offset by a biosynthetic fractionation that is largely constant on thecatchment scale for speciﬁc compounds [Sachse et al., 2012; Thomas
et al., 2012; Shanahan et al., 2013].

We reconstruct precipitation seasonality using δ2H of C24 n-alkanoic acids, produced mainly by aquatic
macrophytes and by bryophytes (δ2Haq, Figure 2 and Figure S4 in the supporting information) [Nichols et al.,2009]. Aquatic plants use lake water as the source for leaf wax production, so δ2Haq changes as a function oflake water δ2H. We calculate Lake N3 water residence time to be 13 ± 3 months (Table S3 in the supporting
information). The lake is not subject to strong evaporative enrichment because it has continuous throughﬂow during the short ice-free season [Jonsson et al., 2009]. Lake water δ2H therefore reﬂects annual mean
δ2Hprecip. This relationship is corroborated by modern aquatic plant n-alkyl lipid δ2H measurements at Lake
N3 (Table S4 in the supporting information). Due to the large seasonal range of δ2Hprecip at Lake N3(Figure 1), an increase in winter snowfall relative to summer precipitation would result in a 2H-depletion ofamount-weighted annual mean δ2Hprecip, and vice versa. Thus, we infer changes in precipitation seasonality
from δ2Haq (Figure 2).
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Geophysical Research Letters10.1002/2016GL068513

We reconstruct qualitative summer
temperature using δ2H of C28 nalkanoic acids, produced by terrestrial plants (δ2Hterr, Figure 3 andFigure S4 in the supporting information). Terrestrial plants likely synthesize leaf waxes at leaf ﬂush, which
occurs in June in modern western
Greenland, and throughout the subsequent growing season [ORNL
DAAC, 2008; Gao et al., 2012; Tipple
et al., 2013]. This relationship is corroborated by modern terrestrial plantn-alkyl lipid δ2H measurements at
Lake N3 (Table S4 in the supporting
information). The vapor sources for
2
Figure 3. (a) Lake N3 δ Hterr (gray shading = 95% age model uncertainty andprecipitation on western Greenland,
vertical black lines = analytical uncertainty). (b) Mean annual snow temBafﬁn Bay, and the Labrador andperature at GISP2 [Kobashi et al., 2011]. Bold black line is a 51 year running
Irminger Seas [Ohmura and Reeh,mean, ﬁne gray line is raw annual data. (c) Midge-inferred average summer
1991; Sodemann et al., 2008] weretemperature anomaly at North Lake [Axford et al., 2013].
ice free during summer throughout
the Holocene [de Vernal et al., 2013;
Ouellet-Bernier et al., 2014], so source areas of summer precipitation for western Greenland likely did not change,
and temperature was the main mechanism inﬂuencing Holocene summer δ2Hprecip. Thus, we interpret δ2Hterr toindicate qualitative centennial-scale changes in Holocene summer temperature on western Greenland.
Terrestrial plant source water in the Lake N3 catchment (soil water) is primarily derived from summer rainfall,but may contain some winter snowmelt, as Arctic soil water residence time is approximately 1 month [Cooper
et al., 1991]. Thus, the orbital-scale changes in precipitation seasonality inferred from δ2Haq likely inﬂuence
δ2Hterr (supporting information). We can use δ2Haq to remove the effect of precipitation seasonality fromδ2Hterr, but this method involves multiple assumptions (Figure S5 in the supporting information). We therefore
interpret only the centennial-scale changes in Lake N3 δ2Hterr and use independent temperature reconstructions to infer orbital-scale terrestrial summer temperature changes on western Greenland [Axford et al., 2013].

3. Holocene Summer Temperature on Western Greenland
Our interpretation of Lake N3 δ2Hterr as summer temperature is supported in several ways (Figure 3). First, δ2Hterr
exhibits similar centennial-scale patterns during the past ~ 300 years as midge-inferred summer temperature
anomalies at North Lake, western Greenland (Figure 3c) [Axford et al., 2013]. Coldest conditions in both records
correspond with the Little Ice Age and end with warming in recent decades, a pattern also documented in Disko
Bugt [Perner et al., 2013]. Second, the centennial-scale variability in Lake N3 δ2Hterr is remarkably similar to the
4 kyr-long record of annual average surface temperature at Greenland Ice Sheet Project 2 (GISP2), Greenland(r2 = 0.43) [Kobashi et al., 2011] (Figure 3b and Figure S5 in the supporting information), suggesting similar controls on late Holocene centennial-scale temperature at these sites, despite seasonal and elevation differences.
The modern summer δ2Hprecip gradient on western Greenland provides an analog for Holocene summer temperature changes recorded by Lake N3 δ2Hterr. Modern summer δ2Hprecip at Kangilinnguit (southern Greenland,
subarctic climate) is ~ 60‰ higher than at Thule (northern Greenland, high Arctic climate) and 20‰ higher
than at Lake N3 (Figure 1 and Figure S3 in the supporting information). Analogously, from 8 to 5 ka, Lake N3
δ2Hterr was 10 to 20‰ higher than during recent decades, indicative of a warm, maritime climate similar tomodern summers at Kangilinnguit (Figure 3a). During the middle to late Holocene, the West Greenland
Current weakened and the sea ice front migrated south, causing cooling on western Greenland (Figure 2)[Perner et al., 2013; Ouellet-Bernier et al., 2014]. This cooling is reﬂected in δ2Hterr, which had values similar to present from 4.5 to 0.3 ka, indicative of an increasingly cold, Arctic climate. The coldest conditions inferred from
Lake N3 δ2Hterr occurred during the Little Ice Age, circa 1600 to 1900 A.D. in this record (Figure 3).
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4. Holocene Precipitation Seasonality on Western Greenland
Lake N3 δ2Haq has a >100‰ range during the past 8 kyr, larger than the range of modern seasonal δ2Hprecip in
Disko Bugt (Figures 1 and 2a). We interpret these large changes in δ2Haq to indicate changes in precipitation seasonality, with Arctic moisture sources contributing to the exceptionally low δ2H values. The magnitude and direction of δ2Haq changes cannot be explained by changes in terrestrial plant contributions of C24 n-alkanoic acids, by
Holocene temperature change, or by lake water residence time (Figures S6 and S7 in the supporting information).
Lake N3 δ2Haq decreased steadily from 190 to 300‰ from 7.5 to 6 ka, in step with declining sea ice in Disko
Bugt (Figures 2a and 2b) [Ouellet-Bernier et al., 2014]. Lake N3 δ2Haq remained low during the middle Holocene,
synchronous with minimum regional sea ice. The Labrador Sea, an important moisture source region for western Greenland [Sodemann et al., 2008], had minimum sea ice cover from 6.5 to 3.5 ka (Figure 2b) [de Vernal et al.,2013]. Furthermore, sea ice reached a minimum in northern Bafﬁn Bay and the Canadian Arctic from 6 to 3 ka
[Ledu et al., 2010; de Vernal et al., 2013], creating a source for moisture that was 2H-depleted compared to modern winter precipitation around Lake N3. Although Bafﬁn Bay still had seasonal sea ice during the middle
Holocene, sea ice would have formed later in the year [Markus et al., 2009]. A loss of sea ice during early winter,when evaporation is greatest, can cause large increases in early winter snowfall [Bintanja and Selten, 2014].
Finally, the West Greenland Current was both strong and warm 6 to 4 ka, likely due to an increased contributionof the warm Irminger Current to the West Greenland Current, resulting in regional warmth and providing
energy to evaporate more moisture (Figures 2c and 2d) [Perner et al., 2013]. Winter insolation was low during
the middle Holocene [Laskar et al., 2004], so there was likely a large contrast between winter air temperature
and winter ocean surface temperature. This temperature contrast would cause high evaporation. Thus, we interpret the signiﬁcant middle Holocene depletion of Lake N3 δ2Haq as an increase in winter precipitation, causedby the near total loss of sea ice cover in the Labrador Sea, and the loss of early winter sea ice and increased oceanic heat advection in Bafﬁn Bay. This relationship has been identiﬁed elsewhere on Greenland. The millennialscale trend of Holocene accumulation at the North Greenland Eemian Ice Drilling Site (NEEM) in northwestern
Greenland tracks, and may have been caused by, changes in sea ice cover in northern Bafﬁn Bay (Figures 2e and2f), supporting our inference that Holocene precipitation trends can be closely linked to sea ice cover [Ledu et al.,2010; de Vernal et al., 2013; Rasmussen et al., 2013].
A brief 2H-enrichment of Lake N3 δ2Haq from 5.2 to 5.0 ka coincides with an abrupt decline in organic matter in
Lake N3 sediments, indicative of reduced primary productivity (Figure S7 in the supporting information). The 2Henrichment may have been caused by brief increases in sea ice cover in the Labrador Sea and Disko Bugt
(Figure 2b) [de Vernal et al., 2013; Ouellet-Bernier et al., 2014]. There is also evidence for a decrease in West
Greenland Current strength and temperature ~ 5 ka, which would decrease heat advection to the region(Figures 2c and 2d) [Perner et al., 2013]. Both of these mechanisms would have caused regional cooling and a
decline in primary productivity, as well as reduced evaporation, causing less snowfall and an increase in δ2Haq.
After 4 ka, δ2Haq increased rapidly to 240‰ at 3.5 ka and then gradually to 210‰ at 2 ka, after which δ2Haq
remained stable (Figure 2a). We interpret this increase in δ2Haq as a decrease in winter precipitation, which coincides with rapid cooling recorded by lakes near Ilulissat [Axford et al., 2013]. Although the West Greenland
Current remained warm until ~ 2 ka, the current weakened at 4 ka, reducing heat advection to Bafﬁn Bay(Figures 2c and 2d) [Perner et al., 2013]. Additionally, gradually increasing winter insolation would have resulted
in a smaller winter ocean-air thermal contrast, reducing evaporation. Evaporation would have been further
reduced by increasing sea ice cover in northern Bafﬁn Bay (Figure 2e) [Ledu et al., 2010; de Vernal et al., 2013].
The increase in northern Bafﬁn Bay sea ice would have cut off the supply of 2H-depleted Arctic moisture to
Disko Bugt [Rasmussen et al., 2013]. Sea ice in Disko Bugt did not increase until ~ 1 ka, after Lake N3 δ2Haq
had reached stable late Holocene values (Figure 2b) [Krawczyk et al., 2013]. The steady decrease in winter snowfall from 4.5 to 2 ka instead likely was caused by increased early winter sea ice cover in northern Bafﬁn Bay andthe Labrador Sea, and cooler regional surface ocean temperatures, phenomena that are likely linked to regionallate Holocene cooling (Figure 3) [Kobashi et al., 2011; Axford et al., 2013; de Vernal et al., 2013].

5. The GrIS Response to Increasing Middle Holocene Winter Precipitation
The middle Holocene increase in winter snowfall recorded by Lake N3 δ2Haq may account for the spatial patternsof Holocene GrIS change. Despite warmer summers, the western GrIS retreated very little during the middle

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Geophysical Research Letters10.1002/2016GL068513

Holocene (Figure 1) [Lecavalier et al., 2014; Young and Briner, 2015]. We hypothesize that increased winter precipitation on the western GrIS during the middle Holocene (Figure 2a) may have offset a portion of ablation at the
GrIS margin caused by warmer summers (Figure 3). A possible exception to this minimal middle Holocene retreat
is the portion of the Greenland Ice Sheet inland of the Sukkertoppen Ice Cap, south of Lake N3, which is the only
portion of the Greenland Ice Sheet modeled to have signiﬁcant (~ 50 km) retreat (Figure 1) [Lecavalier et al., 2014].
This portion of the ice sheet is in an orographic shadow [Ohmura and Reeh, 1991; Bales et al., 2009] and therefore
may not have experienced increased winter precipitation during the middle Holocene, perhaps making this portion of the ice sheet more sensitive to rising summer temperature. Our results at Lake N3 suggest that changes in
the amount of accumulation on the western GrIS are likely controlled by sea ice cover in the Labrador Sea andBafﬁn Bay, and by the strength and temperature of the West Greenland Current.

6. Conclusions
Lipid biomarker δ2H data demonstrate that although western Greenland was warmer during the middle
Holocene, this region experienced increased snowfall. The middle Holocene increase in snowfall was likely
caused by a combination of decreased early winter sea ice cover and increased heat advection to BafﬁnBay. Relatively rapid changes in winter snowfall ~ 5 and 4.5 ka indicate that Arctic hydroclimate is sensitive
to changes in ocean surface conditions, including sea ice and ocean heat content. The response of the western GrIS to higher summer temperatures may have been muted due to increased accumulation in the middle Holocene. Our results suggest that in the future, as Arctic seas warm and sea ice retreats, increased winterprecipitation may enhance accumulation on parts of the GrIS and partly offset summer ablation, particularly
in areas close to modern winter sea ice fronts.
Acknowledgments
This research was supported by aGeological Society of America (GSA)
student research grant, a GSA
Limnogeology Division Kerry Kelts
Research Award, a GSA Quaternary
Geology and Geomorphology Division
Marie Morisawa Research Award, and an
NSF Graduate Research Fellowship to E.
K.T., NSF award 909334 to J.P.B., and NSF
award 0520718 to Y.H. E.K.T. is currently
an NSF EAR Postdoctoral Fellow, NSF
award 1349595. We thank Rafael Tarozo
and Shanna Losee for lab support. LakeN3 chronological and geochemical data
are freely available at the NOAA/World
Data Center for Paleoclimatology(https://ncdc.noaa.gov/paleo/study).

THOMAS ET AL.

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