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INVESTIGATING"THE"UTILITY"OF"ISOTOPE"PROXIES"FOR"SEA(LEVEL"VARIABILITY""
IN"THE"SOUTH"ATLANTIC"OCEAN"
Joanna"C."Peth1,"Chad"S."Lane1,"Andrea"D."Hawkes1,"Jeffery"P."Donnelly2,"Paula"D."Pratolongo3,"&"Eduardo"A."Gómez3"
1"Geography"&"Geology,"University"of"North"Carolina"Wilmington,"2"Geology"&"Geophysics,"Woods"Hole"Oceanographic"Instute,"3"Instuto"Argenna"de"Oceanogra]a"
INTRODUCTION" RESULTS"
ACKNOWLEDGEMENTS"
REFERENCES"
The National Science Foundation (NSF) grant 1154978, awarded to Hawkes and Donnelly, provided funding for this project. In addition, Richard Sullivan of Woods Hole
Oceanographic Institute and students affiliated with the National Scientific and Technical Research Council of Argentina (CONICET-Argentina) provided fieldwork
assistance.
METHODS"
Bentov, S., Brownlee, C., & Erez, J. (2009). The role of seawater endocytosis in the biomineralization process in calcareous foraminifera. Proceedings of the National Academy
of Sciences, 106(51), 21500-21504.
Botto, F., Valiela, I., Iribarne, O., Martinetto, P., & Alberti, J. (2005). Impact of burrowing crabs on C and N sources, control, and transformation in sediments and food
webs of SW Atlantic estuaries. Mar. Ecol. Prog. Ser., 293, 155-164.
Meehl, G. A., Stocker, T. F., Collins, W. D., Friedlingstein, P., Gaye, A. T., Gregory, J. M., Kitoh, A., Knutti, R., Murphy, J. M., Noda, A., Raper, S. C. B., Watterson, I.
G., Weaver, A. J., & Zhao, Z. C. (2007). Global climate projections. Climate change, 3495, 747-845.
Milne, G. A., Long, A. J., & Bassett, S. E. (2005). Modelling Holocene relative sea-level observations from the Caribbean and South America. Quaternary Science Reviews,
24(10), 1183-1202.
Mitrovica, J. X., Tamisiea, M. E., Davis, J. L., & Milne, G. A. (2001). Recent mass balance of polar ice sheets inferred from patterns of global sea-level change. Nature,
409(6823), 1026-1029.
Smith, B. N. & Epstein S. (1971). Two categories of 13C/12C ratios for higher plants. Physiology, 47, 380-384.
Sea-level variability is an indicator of climate change, directly responding to thermal expansion and ice melt (Mitrovica
et al., 2001). Global sea-level rise over the last 100 years is estimated to be between 1–2 mm yr-1 (Milne et al., 2005) and
the most recently published Intergovernmental Panel on Climate Change Assessment Report (IPCC AR4) projected a
global average sea-level rise rate of 19–58 cm yr-1 by the end of the 21st century (Meehl et al., 2007). High-resolution
records of relative sea-level are lacking in the Southern Hemisphere. This project investigates the potential for a multi-
proxy approach to sea-level reconstruction at two salt marsh sites in central Argentina. Salt marshes can be depositional
environments, accumulating centuries to millennia of sediment making them exceptional archives of past sea-level
proxies. The dominant component of salt marsh sediments is derived from autochthonous vascular vegetation that is
generally delineated into strict tidal-inundation-based elevation zones. As the elevation increases in a salt marsh, the
vegetation typically follows a gradient from C4 (salt-tolerant) to C3 species dominance. C3 and C4 plants are associated
with 13C values of -34‰ to -23‰ and -17‰ to -9‰, respectively (Smith & Epstein, 1971). Due to the differing
degrees of isotopic fractionation imparted by plants in each zone, 13C values of sediments often vary across zones.
Similarly, spatial variation in marsh surface water 18O values can exist due to changes in the relative contribution
of 18O-enriched seawater (saline) vs. 18O-depleted meteoric (fresh) waters and potentially recorded in the shells of
calcareous plankton such as foraminifera. Foraminifera undergo extracellular biomineralization – utilizing ions within
the ambient seawater to precipitate their calcite shells (Bentov et al., 2009). Therefore, the isotopic composition of
calcareous foraminifera can be proxies for relative contributions of oceanic vs. meteoric waters.
The overall objectives of this project are to:
• Investigate the potential for a multi-proxy approach to sea-level reconstruction
• Examine the potential for a paleo-environmental signature in marsh calcareous foraminifera
• Examine mechanisms driving ice sheet changes and the contributions of Greenland Ice Sheet (GIS)
• Contribute to the development of a latitudinal gradient of sea-level records along the Atlantic coast
Figure'1.'(A)"Map of southern South America, with location of Bahía Blanca estuary
indicated by the white box labeled B. (B) Map of Bahía Blanca estuary, showing the
regional distribution of the three study sites, Villa del Mar (C), Maldonado Marsh (D)
and General Daniel Cerri (GDC), along the coast. No surface transect was sampled at
GDC, but a core was recovered. (C) Villa del Mar 12-station modern transect from
high marsh to tidal flat, with a core (C1) recovered at station 6. (D) Maldonado Marsh
12-station transect from tidal flat to high marsh, with a core (C1) recovered adjacent to
station 9.
The three salt marsh study sites were strategically selected because they (1) have
small tidal ranges (< 2m) for highest faunal sensitivity; (2) are likely to have high rates
of sea level change over the past several millennia; and (3) are in close proximity to
tide gauges for instrumental – proxy reconstruction comparisons.
Elevaon"(m"MTL)"δ13C"(‰)"δ15N"(‰)"C/N"δ13C"(‰)"δ18O"(‰)"
Elevaon"(m"MTL)"δ13C"(‰)"δ15N"(‰)"C/N"
Figure'4.'Villa"del"Mar"Transect" Figure'5.'Maldonado"Marsh"Transect'
a."Bulk"Sediment"Analysis"
Figure'4'
a. Bulk sedimentary 13C, 15N, and C/N of surface
samples along the Villa del Mar transect. Elevation was
determined by a total station and Real Time Kinematic
(RTK) unit to mean tide level (MTL). Dominant vegetation
zones are noted. Location of core recovery along the
transect is indicated (x).
b. Carbonate 13C and 18O analyses of two species of
calcareous foraminifera, Ammonia beccarrii and Elphidium
spp., along the Villa del Mar surface transect. Location of
core recovery along the transect is indicated (x).
Figure'5'
Bulk sedimentary 13C, 15N, and C/N of surface
samples along the Maldonado Marsh transect. Elevation
was determined by a total station and Real Time Kinematic
(RTK) unit to mean tide level (MTL). Dominant vegetation
zones are noted. Location of core recovery along the
transect is indicated (x).
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(25" (20" (15" (10" 4" 6" 8" 10" 12" 4" 6" 8" 10" 12"
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(3.5" (2.5" (1.5" (0.5" 0.5" (1.5" (0.5" 0.5" 1.5"
δ13C"(‰)" δ15N"(‰)" C/N" δ18O"(‰)"δ13C"(‰)"
δ13C"(‰)" δ15N"(‰)" C/N"
Depth"(cm)"Depth"(cm)"
δ13C"(‰)" δ15N"(‰)" C/N"
Depth"(cm)"
Figure'6.'Villa"del"Mar"Core"
a."Bulk"Sediment"Analysis" b."Calcareous"Foraminifera"Analysis"
Figure'7.'Maldonado"Marsh"Core"
Figure'8.'General"Daniel"Cerri"Core"
DISCUSSION"
Modern'and'Fossil'Sediment'Sampling"
Sediment cores were recovered at each site and stratigraphy described based on color and textural changes.
Foraminifera tests and bulk sediments were sub-sampled throughout each core for isotopic analysis (see below).
Additionally, 12-station surface transects were sampled perpendicular to shore at Villa del Mar and Maldonado Marsh
to establish modern isotopic trends across the elevational gradient.
Due to the highly evaporative environment of central coastal Argentina, the chosen salt marsh sites are unique
compared to sites typically chosen for sea-level reconstruction studies. As a consequence of the high salinity conditions,
the calcareous foraminifera that are typically restricted to the low marsh can inhabit the entire transect. This allows an
isolation of monospecific foraminifera tests for isotopic variability with elevation.
δ13C,'δ15N,'C/N'Analysis'of'Bulk'Sediments"
Core and surface samples were dried at 50 °C and rinsed with HCl to remove excess carbonate. Cores were sampled at
10 cm intervals. Samples were analyzed on a Costech 1040 Elemental Analyzer interfaced with a Thermo Delta V Plus
stable isotope ratio mass spectrometer (EA-IRMS) versus standards, USGS 40 and USGS 41. Precision of these
analyses was better than 0.3‰.
δ18O,'δ13C'Analysis'of'Calcareous'Foraminifera"
Fossil and modern foraminifera tests were isolated by wet-sieving sediments through 500, 180, and 63- m meshes.
Cores were sampled at 10 cm intervals. Strictly Ammonia beccarrii, Elphidium excavatum clavatum, and Elphidium
subarcticum calcareous benthic foraminifera were sampled and analyzed on a Finnigan Gas Bench II interfaced with a
Thermo Finnigan Delta V Plus stable isotope mass spectrometer. Samples were standardized to NBS 16 and NBS 18.
Precision of these analyses was better than 0.3‰.
Ammonia'beccarrii'
Elphidium'excavatum'clavatum''
Elphidium'subarc6cum''
Figures'6'a.,'7,'8''
Bulk sedimentary 13C, 15N, and C/N of
core samples from the Villa del Mar,
Maldonado Marsh, and General Daniel
Cerri study sites. Fossil mollusk shells were
radiocarbon dated along the Villa del Mar
and Maldonando Marsh cores.
'
Figure'6'b.''
Carbonate 13C and 18O analyses of two
species of calcareous foraminifera, Ammonia
beccarrii and Elphidium spp., in the Villa del
Mar sediment core.
S.'densiflora'Tidal" Tidal"
• The sedimentary 13C values fall between the ranges for C3 and C4 terrestrial plants. This may be due to a mixing
of C3 and C4 organic matter at the core site or a dominance of marine algae and/or marine-derived organics in the
marsh sediments.
• The 15N values are elevated in both the cores and surface sediments. This signature may be due to the high
degree of denitrification that takes place in marsh environments, especially those with abundant crab burrows
(Botto et al., 2005), thereby increasing the relative abundance of the heavier 15N isotope.
• An increase in 13C value of bulk sediments at the top of the core is present at all 3 sites. The dense cover of
Spartina alterniflora at Maldonado Marsh is associated with elevated 13C values, thus an increase in 13C may
indicate increased S. alterniflora (due to higher sea levels) at the core site in the last few hundred years. The lower
marsh biomass cover at Villa del Mar may make the bulk 13C record less sensitive to vegetation change.
• C/N ratios of bulk sediments are lower than expected for vascular vegetation (typically >20). This may be due to
an abundance of microbial biomass (denitrifying bacteria) and passive deposition of detritus from crabs.
• Increased 18O values of foraminifera tests in the upper ~40 cm of the Villa del Mar sediment core indicate a
recent increase in salinity likely related to sea level rise since 479-520 cal yr B.P. The divergent (decreasing) 13C
trend through time may be the result of a changing DIC pool caused by increased DOM inputs from the
developing marsh ecosystem.
No"veg."
(1.5"
(1"
(0.5"
0"
0.5"
1"
1.5"
2"
0" 50" 100" 150" 200" 250" 300" 350" 400"
b."Calcareous"Foraminifera"Analysis"
(4"
(3.5"
(3"
(2.5"
(2"
(1.5"
(1"
(0.5"
0"
Ammonia'beccarrii'
Elphidium'spp.'
0"
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12"
3"
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11"
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(21"
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0" 50" 100" 150" 200" 250" 300" 350" 400"
S.'alterniflora'Sarcocornia'
spp.'
Sarcocornia'
spp.'
S.'alterniflora'
(1"
(0.5"
0"
0.5"
1"
1.5"
(1"
(0.5"
0"
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1.5"
GDC
62.2 W62.4 W
39 S
38.8 S
38.6 S
38.4 S
N
km
0 52.5
ooo
o
o
o
o
62 W
Bahía Blanca
Estuary
Bahía Blanca
Punta
Alta
C
D
B
0 5025
m
N
Bahía Blanca
Estuary
C1
12
11
10
9
8
7 6 5
4 3 2 1
C'
C1
0 5025
m
N
Bahía Blanca
Estuary
12
11
10
9
8
7
65
43
2
1
D
STUDY"SITE"
km
0 400200
Argentina
Chile
Uruguay
Brazil
Falkland Islands
Atlantic Ocean
_ 50˚S
_
_
_
_
_
_
_
_
_
_
_ 52˚S
40˚S
42˚S
44˚S
46˚S
48˚S
38˚S
36˚S
34˚S
32˚S
30˚S
II I
70˚W
IIIIIIIII I II
62˚W66˚W74˚W78˚W 50˚W54˚W58˚W
N
B
Pacific Ocean
A
= Land = Marsh= Cities & Roads = Upland Marsh = Water
Figure'2.'Photo'of'Villa'del'Mar'
Note the patchy vegetation cover relative to Maldonado Marsh.
Figure'3.'Photo'of'Maldonado'Marsh'
Note the dense Spartina spp. cover relative to the other sites.
Figure'9.'General'Daniel'Cerri''
Note the patchy vegetation cover and salt deposits due
to the highly evaporative environment.
500'±'20'
(cal'yr'B.P.)'
856'±'60'
(cal'yr'B.P.)'
2378'±'44'
(cal'yr'B.P.)'
3413'±'37'
(cal'yr'B.P.)'
0 50" 100" 150" 200" 250" 300" 350" 400"
Distance"Along"Transect"(m)" Distance"Along"Transect"(m)"
Distance"Along"Transect"(m)"