Climate Forcing of the Terrestrial Organic Carbon Cycle During the Late Deglaciation:The Himalaya-Bengal Fan Example

Christopher J. Hein1 (hein@vims.edu), Valier Galy2 (vgaly@whoi.edu), Hermann Kudrass3 (kudrass@gmx.de), Timothy I. Eglinton4 (timothy.eglinton@erdw.ethz.ch), Bernhard Peucker-Ehrenbrink2 (bpeucker@whoi.edu) 1Department of Physical Sciences, Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA, USA; 2Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA USA;

3MARUM - Zentrum für Marine Umweltwissenschaften Universität Bremen Leobener Straße 28359 Bremen, Germany; 4Geological Institute, Department of Earth Sciences, Sonneggstrasse 5, ETH, 8092 Zurich, SWITZERLAND

Abstract ID: 248259

5. Monsoon Intensity, Paleo-vegetation, and Sediment / OC Burial Since LGM

4. Sediment Contributions to the Bay of Bengal Since LGM

1. Climatic Forcing of Carbon Cycle Dynamics

3. Record Location: Ganges-Brahmaputra Basin / Bay of Bengal

9. Acknowledgments

6. Climatic Control ofOrganic Carbon Residence

Time in the G-B Basin

2. Organic-Carbon Export Dynamics in the

Ganges-Brahmaputra Basin

8. References

7. Implications

Contreras-Rosales, L.A., Jennerjahn, T., Tharammal, T., Meyer, V., Lückge, A., Paul, A., Schefuß, E., 2014. Evolution of the Indian Summer Monsoon and terrestrial vegetation in the Bengal region during the past 18 ka. Quaternary Science Reviews, v. 102, p. 133-148.

Galy, A., France-Lanord, C., 2001. Higher erosion rates in the Himalaya: geochemical constraints on riverine fluxes. Geology, v. 29, p. 23–26.

Galy, A., France-Lanord, C., Derry, L.A., 1999. The strontium isotopic budget of Himalayan rivers in Nepal and Bangladesh. Geochimica et Cosmochimica Acta, v. 63, p. 1905-1925.

Galy, V., Eglinton, T.I., 2011. Protracted storage of biospheric carbon in the Ganges-Brahmaputra basin. Nature Geoscience, v. 4, p.843-847.

Galy, V., France-Lanord, C., Beyssac, O., Faure, P., Kudrass, H., Palhol, F., 2007. Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system. Nature, v. 450, p. 407–410.

Galy, V., François, L., France-Lanord, C., Faure, P., Kudrass, H., Palhol, F., Singh, S.K., 2008. C4 plants decline in the Himalayan basin since the last glacial maximum. Quaternary Science Reviews, v. 27, p. 1396–1409.

Galy, V., France-Lanord, C., Peucker-Ehrenbrink, B., Huyghe, P., 2010. Sr-Nd-Os evidence for a stable erosion regime in the Himalaya during the past 12 Myr. Earth and Planetary Science Letters, v. 290, p. 474-480.

Galy, V., France-Lanord, C., Beyssac, O., Lartiges, B., Rhaman, M., 2011. Organic Carbon Cycling During Himalayan Erosion: Processes, Fluxes and Consequences for the Global Carbon Cycle. in: Lal, R., Sivakumar, M.V.K., Faiz, S.M.A.M.A., Mustafizur Rahman, A.H.M.H.M.M., Islam, K.R.R. (eds.), Climate Change and Food Security in South Asia. Netherlands: Springer, p. 163-181.

Goodbred, Jr., S.L., and Kuehl, S.A., 2000. Enormous Ganges-Brahmaputra sediment load during strengthened early Holocene monsoon. Geology, v. 28, p. 1083-1086.

Herzschuh, U., 2006. Palaeo-moisture evolution in monsoonal Central Asia during the last 50,000 years. Quaternary Science Reviews, v. 25, p. 163-178.

Kudrass, H.R., Hofmann, A., Dosse, H., Emeis, K.-C., Erlenkeuser, H., 2001. Modulation and amplification of climatic changes in the Northern Hemisphere by the Indian summer monsoon during the past 80 k.y. Geology, v. 29, p. 63-66.

Lupcker, M., France-Lanord, C., Galy, V., Lave, J., and Kudrass, H., 2013. Increasing chemical weathering in the Himalayan system since the Last Glacial Maximum. Earth and Planetary Science Letters, v. 365, p. 243-252.

Singh, S., France-Lanord, C., 2002. Tracing the distribution of erosion in the Brahmaputra watershed from isotopic compositions of stream sediments. Earth and Planetary Science Letters, v. 252, p. 645-662.

Weber, M.E., Wiedicke, M.H., Kudrass, H.R., Hübscher, C., Erienkeuser, H., 1997. Active growth of the Bengal Fan during sea-level rise and highstand. Geology, v. 25, p. 315-318.

This work was funded by NSF OCE Grants 133826 (Virginia Institute of Marine Science), 1333387 (Woods Hole Oceanographic Institu-tion; WHOI), and 0928582 (WHOI). C. Hein was partially funded by the WHOI Coastal Ocean Institute Postdoctoral Scholar program. Significant analytical contributions and labo-ratory assistance were provided by X. Philip-pon, C. Johnson, S. Sylva & K. Fornace of WHOI & A. McNichol & the staff of the Na-tional Ocean Sciences Accelerator Mass Spec-trometry Facility (NOSAMS).

Glacial Conditions

Atmosphere

Bay of Bengal

Terrestrial Vascular Plants

Ganges-Brahmaputra River

Enhanced OC Storage in decadal Reservoir Decadal OC

Reservoir (organic litter)

Millennial OC Reservoir

(soils, floodplains, riverbed, terraces)

Holocene Conditions

Atmosphere

Decadal OC Reservoir

(organic litter)

Bay of Bengal

Terrestrial Vascular Plants

Ganges-Brahmaputra River

Millennial OC Reservoir

(soils, floodplains, riverbed, terraces)

A) B)

Feedbacks: Climate Change & OC-Export Dynamics (Fig. 7):Negative correlation between monsoon strength & OC RT → back-ground millennial contribution with variable decadal contribution.Glacial Period (Fig. 7a): weak monsoon → lower productivity and / or enhanced storage of OC in millennial stores → long OC RT.Holocene (Fig. 7b): warmer & strengthened monsoon → enhanced export of decadal OC (higher percentage of total OC export) & enhanced fixation of decadal OC → decadal OC reservoir increases → increased sequestration of atmospheric CO2 → negative feedback.

Fig. 7. Climate / carbon-cycle feedbacks Simplified box models showing negative feed-backs between monsoon strength and terrestrial OC storage.

Weighted Average C24-32 FA

C24-32 FA Regression Line

Explanation

y = 0.0035x - 177R2: 0.728

y = 0.0043x - 183R2: 0.763

C28 FA

C28 FA Regression Line

Strong Monsoon

Weak Monsoon

-150

-145

-155

-160

-165

-170

-175

-180

-185

-190

Residence Time (yr)0 1000

-140

2000 3000 4000 5000 6000 7000 8000 9000

Ter

rest

rial L

eaf W

ax F

atty

Aci

d

δD (‰

VS

MO

W)

Fig. 6. Climatic control of OC residence time in the G-B River Basin. Comparison of stable hydrogen isotopic compo-sitions (δD) and RT of terrestrial leaf wax FA. Note: errors within data points if error bars not shown.

Climate Change Impacts on OC-Export Dynamics (Fig. 6):RT calculated as difference between deposition age of C24+FA (from foram ages or core age model) & calibrated age of C24+FA .Negative correlation between monsoon strength and RT of organic carbon in G-B system.Preliminary data: ca. 250 yr change in RT per 1 ‰ change in FA δD.Indicates likely mixing of millennial & decadal OC inputs.

FA δD → decadal signal buffered by background millennial signal.Wetter climate → higher decadal input → shorter RT.

Fig. 5. Records of climate changes, vegetation changes, and OC inputs to the BoB since LGM. Errors are within data points if error bars not shown.

A) Climate change in the G-B / Bengal Basin since LGM. (i) “Effective moisture” in central Asia (Herzschuh, 2006).(ii) 3-pt. moving average of δ18O of planktonic forams (G.ruber) from BoB channel-levee

cores SO93-117KL, 118KL, & 120KL (data: Weber et al., 1997 & Galy et al., 2008).(iii) BoB sea-surface salinity, as derived from foram δ18O record(Kudrass et al., 2001).(iv) Sediment hydration, a proxy for terrestrial chemical weathering, from the Bengal shelf

(cores SO93-105KL & 107KL) and channel-levee system (cores SO93-117KL, 118KL, & 120KL) (Lupker et al., 2013).

(v) Compound-specific hydrogen isotope compositions (rainfall intensity) of plant wax n-alkanes from Bengal Shelf core SO188-342KL, sourced from Indo-Burman (I-B) range with possible contributions from G-B basin (Contreras-Rosales et al., 2014).

(vi) Compound-specific hydrogen isotope compositions of plant wax n-alkanoic (fatty) acids from channel-levee cores, sourced solely from G-B basin (this study).

B) Paleovegetation in the G-B Basin since LGM. (i) Compound-specific δ13C compositions (vegetation type sources) of plant wax n-alkanes

from Bengal Shelf core SO188-342KL (Contreras-Rosales et al., 2014).(ii) 3-pt. moving average of bulk δ13C channel-levee cores (Galy et al., 2008; this study). (iii) Compound-specific δ13C compositions of plant wax n-alkanoic (fatty) acids from

channel-levee cores (this study).

B) Carbon loading in the Bay of Bengal. (i) Total organic carbon content from channel-levee cores (this study).(ii) Concentration (as a function of sediment mass) of long-chain (C24+ ), terrestrially

derived fatty acids from channel-levee cores (this study).(iii) Updated sedimentation rate record for channel-levee core SO93-120KL based on

calibrated foram radiocarbon ages (data from Weber et al., 1997; this study).

Climate Change in the G-B Basin since LGM (Fig. 5a):Last Glacial Maximum (LGM) to Holocene Climatic Optima (HCO): Northern Hemisphere insolation increases → monsoon activity increases → moisture in G-B Basin increases. BoB salinity decreases (decrease in δ18O of planktonic forams in sediment cores) & weathering in G-B Basin increases.Plant wax (n-alkanes & fatty acids [C24+FA]) δD record increase in monsoon strength; channel-levee C28 FA (this study) show 45 ‰ shift from H1 to HCO.Shelf & channel-levee records largely similar; shifts smaller in shelf record →possible signal buffering by Indo-Burman inputs to shelf record.Trends slighly reversed since mid Holocene.

Vegetation Changes in the G-B Basin since LGM (Fig. 5b):3 ‰ shift in δ13C of bulk OC: transition of vegetation, C4 to C3 dominance, since LGM (Galy et al., 2008).~5 ‰ shift in compound-specific δ13C of channel-levee C28 FA (this study) LGM to HCO.Compound-specific n-alkane (Bengal shelf) & C24+FA (channel-levee system) records show very similar trends.Muted reversal in vegetation type since HCO.Outlier: temporary C3 excursion during Bølling-Allerød? Source effect?

Sediment & OC Burial in the BoB since LGM (Fig. 5c):No trends in OC burial or loading since LGM; C24+FA concentrations are consistent with G-B river sediments → high OC burial efficency.Large peaks in sediment burial in channel-levee system during & following Younger Dyras (similar to Goodbred & Kuehl, 2000).

Gange

s

Brahmap

utra

Lower

Meghn

a

C)

Age (kyr B.P.)0 2 4 6 8 10 12 14 16 18 20

Youn

ger

Dry

asB

øllin

g-A

llerø

dHolocene Climatic Optimum

Late GlacialA)

B)

i.

ii.

iii.

iv.

v.

vi.

i.

ii.

iii.

i.

ii.

iii.

H1

2.0

6.0

4.0

0.0

Terrestrial OC

Loading ([C

24+ ], mg/gS

ed)

Low Loading

High Loading

Sed

imen

tatio

n R

ate

(m

m/y

r)

2.0

4.0

0.0

3.0

5.0

1.0

Slow Deposition

Rapid Deposition

Car

bon

Load

ing

(T

OC

, %) 0.8

0.6

0.4Low C Loading

High C Loading

-31C3 Plants (forest)

C4 Plants (savanna)

Ter

rest

rial L

eaf W

ax

Fa

tty A

cid

δ1

3 C (‰

VP

DB

)

-23

-27

-29

-25

Outlier (?)

-20.0

-23.0

-22.0

-21.0

-19.0

-18.0

Bulk O

rganic Carbon

δ13C

(‰ V

PD

B)

C3 Plants

C4 Plants

-24.0

Weighted Ave. C24-32 FAC28 FA

-29

-25

-27

-23

C3 Plants (forest)

C4 Plants (savanna)

Ter

rest

rial L

eaf W

ax

Wt.

Ave.

C29

& C

31 n

-alk

ane

δ13 C

(‰ V

PD

B)

-160

-140

-150

-170

Terrestrial Leaf Wax Fatty A

cid δD

(‰ V

SM

OW

)

Weak Monsoon

Strong Monsoon

-180

-190

-200

Weighted Ave. C24-32 FAC28 FA

-130

-160

-140

-150

-170

Ter

rest

rial L

eaf W

ax

Wt.

Ave.

C29

& C

31 n

-alk

ane

δD (‰

VS

MO

W)

Weak Monsoon

Strong Monsoon

Chem

ical Weathering

(H2 O

+/Si*, m

ol:mol)

0.09 Reduced

0.10

0.11

0.12

0.13

0.14

Bengal ShelfFan Channel-Levee

Enhanced

High Freshwater Inputs

Low Freshwater Inputs

Sal

inity

(‰)

32

35

34

33

31

δ18O

(‰V

PD

B)

-2.5

-0.5

-1.0

-1.5

-2.0

-3.0

Mea

n E

ffect

ive

Moi

stur

e

2.0

0

0.5

1.0

1.5

2.5Wet

Dry

Age (ka B.P.)

εNd

87S

r/86S

r

1 3 5 7 9 11 13 15 17 19

0.72

0.78

-20

-18

-16

-14

Modern Rivers

21

0.76

0.74

Holocene Climatic Optimum

Late Glacial

Youn

ger

Dry

as Explanation

BoB Shelf BoB Channel-Levee System

CoresCores

Modern RiversModern RiversHimalayan Front Rivers

Ganges River

Brahmaputra River

Confluent G-B River

Bøl

ling-

Alle

rød H1

Fig. 4. Sr & Nd isotopic composi-tion of modern & Bengal Fan sediments (modified from Lupker et al., 2013). Channel-levee cores are the same as those used in this study (117KL, 118KL, 120KL). River-sediment analyses compiled, in part, from Galy et al. (1999) and Singh & France-Lanord (2002).

OC exported by each G-B rivers has significantly-distinct composition (Galy & Eglinton, 2011; Galy et al., 2011).

Himalayas have unique Sr-Nd-Os signature compared to other local potential sediment sources (Galy et al., 2010).

OC in Ganges Basin has shorter average RT & greater contribution of C4 plants than Brahmaputra Basin → need to account for changes in sediment & OC sources.

Stable isotopic (Sr & Nd) compositions over 21 kyrs stable & consistent with modern G-B mixing (Fig. 4).

bulk biospheric OCC24+ FA

Confluent G-B

Brahmaputra River

suspended sedGanges River

bedload sed

suspended sedbedload sed

∆14 C

(‰)

0

+100

-100

-200

-300

-400

0.1

1.0

2.5

5.0

Average age (kyrs.)

lignin phenols

Fig. 2. Ages of components of biospheric OC in sediments collected from G-B rivers. Error bars: 95% confidence interval; errors are within points where absent. C24+ FA & bulk biospheric data from Galy & Eglington (2011); lignin phenol data are unpublished.

Ganges-Brahmaputra (G-B) Rivers & Bay of Bengal:~20% of global terrestrial biospheric OC burial (~3.9 x 1011 mol/yr; Galy et al., 2007, 2011).

Age of biospheric OC Exported from G-B Rivers: Bulk biospheric OC (“fresh” + “aged”): 3000 yrs. “Aged” OC: >17 kyrs. - millennial OC “Young” OC: 50-1300 yrs. (decadal OC) - mix of compounds with different relative RTs, such as: ◦ Lignin-derived phenols: < 50 - ~100 yrs.◦ n-alkanoic (fatty) acids (C24+ FA): ~400 yrs.

Bengal Fan

Himalaya

Brahmaputra

Ganges

Tibetan plateau

117KL

120KL

118KL

Shelf

activ

e ch

anne

lApproximate

Fan BoundaryArea of Detail

inner levees

117KL

118KL

120KL (projected)

outer levee

West Eastactive channel

2540

2560

2580

2600

2620

2640

outer levee

10 km

Depth (m

below sea level)

20o N

0o

80o E 90o E

World’s largest fluvial sediment load (~109 t/yr; Galy & France-Lanord, 2001).Sediment & organic carbon delivered to middle & lower fan in Bay of Bengal (BoB) via 2500-km long channel-levee system (Fig. 3).Sediment cores (117KL, 118KL, 120KL) from channel-levee record last 20 kyrs of sedimentation and OC export from G-B rivers.Sediment core age models provided by 14C dating of planktic foraminifera (G. ruber & G. sacculifer) (Weber et al., 1997; this study).

Fig. 3. Study area - G-B Basin and Bengal Fan. Red: sediment cores used in study. Seismic-reflection profile from mid-fan channel-levee of Bengal Fan (Weber et al., 1997).

Atmosphere

Decadal OC Reservoir

(organic litter)

Marine Sediments

Terrestrial Vascular Plants

Rivers, Lakes

Millennial OC Reservoir

(soils, floodplains, riverbed, terraces)

Atmospheric CO2

Export to Marine Sediments

Remineralization to Atmosphere

Transfer of “Aged” Millennial OC to

Rivers

Transfer of “Young” Decadal

OC to Rivers

A) B)

Fig. 1. Conceptual Model. (a) major pathways of carbon exchange be-tween atmosphere, terrestrial vegetation, terrestrial “decadal” and “millennial” OC reservoirs and rivers. (b) simplified box model of OC transfer between atmospheric, terrestrial & marine reservoirs.

Atmospheric CO2 affected by small changes in size & residence time (RT) of carbon in other reservoirs (terrestrial, oceans, etc). Terrestrial organic carbon (OC) reservoir subdivided into 3 pools: (1) “Petrogenic” (bedrock-derived); (2) “Aged” (Millennial) (stored in soils, floodplain, etc.); (3) “Young” (Decadal) (rapid turnover & export to ocean)Climate change affects cycling between terrestrial pools and timescales & sources of OC export to other reservoirs.