










































  ARESTY  RUTGERS UNDERGRADUATE RESEARCH JOURNAL, VOLUME I, ISSUE II 
 
 
 

 

 

 

[a] School of Earth and Environmental Sciences, Queens College, CUNY, Flushing, NY 
[b] Department of Earth, Atmospheric, and Planetary Sciences, MIT, Cambridge, MA 
 
 

 
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. 

INVESTIGATING THE 
STRENGTH OF THE  

INDIAN MONSOONS 
DURING CLIMATE  
EXTREMES WITH  
STABLE ISOTOPE  

RECORDS IN CORALS 

HANNAH VARKEY,  RICHARD MORTLOCK (FACULTY ADVISOR),  
CECILIA M. MCHUGH[a], DHIMAN R. MONDAL[b] 

 
 
 

✵ ABSTRACT  

The Indian monsoon affects the lives of over 
a billion inhabitants living in southern Asia via the hy-
drological cycle. Agriculture on land and freshwater 
discharge into the ocean. This discharge and nutri-
ent cycling are tied with the monsoon cycles that di-
rectly impact society and the economy. Previous 
studies have demonstrated a strong connection be-
tween the strength of the Indian monsoon and the 
cooling of the North Atlantic during climate ex-
tremes, such as during the last glacial period 20,000 
years ago, and the Little Ice Age (~1300-1870 A.D.). 
In our study, we compare the relative strength of the 
monsoon during two different climate states: the Lit-
tle Ice Age (LIA) and the modern (2015) with proxy 
measurements obtained in surface corals from Saint 
Martin’s Island, Southeast Bangladesh. We used the 
oxygen-isotope 18O/16O ratio (δ18Oc) of coralline 
aragonite (CaCO3) to reconstruct changes in the 
δ18O of seawater (δ18Ow) attributed to freshening 
from monsoon rains. During both climate states, 

corals recorded large variations in δ18Oc (up to 2 
parts per thousand or ‰). We attribute these 
changes, in part, to local salinity changes which are 
reflected by variability in δ18Ow from local riverine 
discharge. While our records only represent 5-year 
snapshots and may not be representative of the av-
erage climate state, this data does not support that 
the monsoon was substantially weaker during the 
LIA compared to the modern. In this study, the coral 
records indicate subtle patterns of isotopic compo-
sition as a function of precipitation and temperature 
variability, serving as a preliminary for further study 
through longer records lasting a century. Beyond 
this, it would better our understanding of interac-
tions between extremes in temperature and climate 
systems. 

 

1 INTRODUCTION 
Saint Martin’s Island, Bangladesh lies in the 

heart of the Indian monsoon (FIGURE 1) where seasonal 
shifts in wind direction bring torrential rainfall. The 
summer monsoons, seasonal wind, and rains 
brought about by differential heating of the land and 
water, last from June to September every year. Dur-
ing the winter, the winds reverse towards the south-
west direction, and precipitation is reduced. The 
seasonal cycle of the monsoon influences local sea-
water hydrography in two important ways. During 
the summer, sea surface temperature (SST) in-
creases, bringing about an increase in rainfall and lo-
cal riverine input which serves to lower salinity. The 
freshening of seawater leads to a decrease in δ18O in 
seawater (δ18Ow) since precipitation has a much 
lower δ18O as a result of the Raleigh Distillation pro-
cess[3]. Oxygen isotopes undergo fractionation pro-
cesses, where water containing the lighter 16O iso-
tope is more likely to get evaporated to form a gas-
eous or vapor state and then precipitated as liquid in 
rain. Hence, the freshening of seawater via precipita-
tion results in a positive correlation between δ18Ow 
and salinity, as both are lowered. 



  ARESTY  RUTGERS UNDERGRADUATE RESEARCH JOURNAL, VOLUME I, ISSUE II 
 
 
 

Reliable instrumental records of SST and sa-
linity are scarce beyond the early 20th century. Cor-
als, however, provide a means for reconstructing sur-
face water conditions beyond instrumental records 
because the δ18Oc recorded by the corals depends 
upon both the δ18O of the surrounding seawater and 
its temperature[7]. Fractionation of 16O and 18O in-
creases with decreasing temperature so that higher 
δ18Oc is associated with lower SSTs and lower δ18Oc 
is associated with higher SSTs. Many corals display 
annual banding patterns that reflect changes in the 
density of the skeletal material. When x-rayed, the 
corals exhibit couplets of light and dark bands. Each 
dark-light couplet represents a one-year growth. 
With high resolution micro-milling, one can often 
obtain samples at monthly resolution for recon-
structing past changes in salinity and SST. 

Here, we compare the strength of the mon-
soon during two different climate states: the Little Ice 
Age (LIA, ~1300-1870 A.D.) and the modern (died in 
2015) by comparison of their stable isotope and 
trace metal chemistry. It is hypothesized that the 
monsoon should have been weaker during the LIA. 
Satellite data suggests St. Martin’s Island does not 
experience large seasonal variations in SST (range of 
less than 3°C). Therefore, large changes in δ18Oc 
should be driven by changes in δ18Ow with lower 
δ18Ow recorded during the summer monsoon. Sea-
sonal variation in SST can also modify δ18Oc. In order 
to constrain this seasonal variability in SST, we pre-
sent measurements of coral Sr/Ca, which has been 
shown to be a reliable recorder of temperature[3].  
Finally, we use the carbon-isotope ratio of 13C to 12C 
(δ13Cc) as an indicator of coral feeding strategy  
(photosynthesis vs heterotrophy) and δ13C of the dis-
solved inorganic carbon pool (DIC) in seawater. 
 

2 METHODOLOGY 
Both LIA and modern corals in this study belong to the Porites species, which are stony corals with small 

polyps. Porites are important in paleoclimatology studies, frequently used as recorders of past marine conditions. 
The corals tend to be grey-brown to white in color and form hemispherical mounds or ‘microatolls’ in intertidal 
zones in the Indo-Pacific waters[4]. They can have greenish tints to the outer walls, due to their symbiotic relation-
ship with single-celled zooxanthellae within the tissues, or more specifically, corallites. Corallites, skeletal cups 
formed from each polyp, are composed of calcium carbonate and precipitated as the mineral aragonite. The 
corals were collected during sampling expeditions to St Martin’s Island in 2015/2016. At present, they are stored 
in airtight containers and drilled using a micro-mill to collect samples along a transect. 

FIGURE 1: Saint Martin’s Island: the location of LIA coral 
D09-01corresponds to SM-D09. The modern coral loca-
tion corresponds to that of SM-Q07[4] 



  ARESTY  RUTGERS UNDERGRADUATE RESEARCH JOURNAL, VOLUME I, ISSUE II 
 
 
 

 

  

  
To identify the banding patterns and to 

guide micro-milling, x-rays of the slabs were taken at 
the Radiology Lab at Robert Wood Johnson Univer-
sity Hospital, New Brunswick. We micro-sampled the 
slabs at 0.5 to 1 mm spacings parallel to the growth 
axis using a manual drill. Annual growth bands are 
~1 cm wide and suggest our sampling resolution is 
monthly. Approximately 80 μg of each powdered 
sample was analyzed by stable isotopic mass 

spectrometry in the stable isotope facility in the De-
partment of Earth & Planetary Sciences at Rutgers 
University. Isotope data was reported relative to PDB 
(Pee Dee Belemnite, the standard established for 
δ18O and δ13C) in the standard per mil (‰) notation  
(EQUATION 1). Measurement precision (1 SD) is 0.08‰ 
for δ18O and 0.05‰ for δ13C, respectively. For the 
strontium-calcium (Sr/ Ca) analysis, an adjoining 
transect of D09-01 was micro-milled and sampled at 
a similar spacing. ~70 μg of coral powder was acidi-
fied to 400 microliters of 3% nitric acid and analyzed 
by an inductively coupled plasma atomic emission 
spectrophotometer or ICP-OES for Sr/Ca isotopic ra-
tios (at the Dept. of Marine and Coastal Sciences), 
similar to methods in Schrag, 1999[6]. Sr/Ca ratios 
were converted to SST via EQUATION 2, demonstrating 
a temperature sensitivity of about 2 °C for a change 
of 0.1 in Sr/Ca. 

ABOVE: 
FIGURE 2: Sampled D09-01 Porites lobata coral 

BELOW: 
FIGURE 3: Sampled Living Porites lutea coral 

 

EQUATION 1: δ18O is the ratio of stable isotopes oxygen-18 (18O) to oxygen-16 (16O), in a sample relative to the ratio in a standard. 
It is defined in “per mil” (‰, parts per thousand) 

EQUATION 2: Porites Sr/Ca = 10.790 (±0.043)– 0.068 (±0.002) x SST (°C), where the Sr/Ca ratio is expressed in mM/M units.[5] 

(1) 
 

 

(2) 



  ARESTY  RUTGERS UNDERGRADUATE RESEARCH JOURNAL, VOLUME I, ISSUE II 
 
 
 

3 RESULTS 
FIGURE 4 and FIGURE 5 show stable isotope re-

sults in the two corals— fossil D09-01 (U-Th dated to 
1762) and modern (collected in 2015), respectively. 
Oxygen isotopic ratios, δ18O and carbon isotopic  
ratios δ13C were primary indicators of past sea  
conditions. The δ18O is a function of temperature 
and salinity, while δ13C measures productivity and 

increased photosynthesis from vegetation and or-
ganisms that have a symbiotic relationship with cor-
als. This relationship correlates with the amounts of  
sunlight received over time. 

Repeating patterns of high and low δ18O and 
δ13C are associated with the banding patterns in 
both the LIA and modern corals (FIGURE 4 and FIGURE 5). 
These patterns suggest that the changes in isotopic 

 
ABOVE: 
FIGURE 4: LIA coral D09-01. Note that δ18O and δ13C variations are aligned with the high-density and low-density banding 
displayed in the X-ray image. The red lines plot the δ18O variations and the blue lines, δ13C.  

BELOW: 
FIGURE 5: δ18O and δ13C variations shown alongside an X-ray image in the modern coral. 

 



  ARESTY  RUTGERS UNDERGRADUATE RESEARCH JOURNAL, VOLUME I, ISSUE II 
 
 
 

values are driven by seasonal changes. The banding 
patterns suggest both coral records represent 5 
years growth. Both the mean and range in δ18O in 
the modern coral is similar to that of the LIA coral  
(~ 1.2‰). The mean LIA coral δ18O is higher, by 0.2‰ 
compared to the modern sample, although differ-
ences in the mean values are not statistically signifi-
cant (TABLE 1 and TABLE  2). The range in δ13C of the liv-
ing coral are similar to those in the LIA coral, about 
3‰. The modern coral average, however, is about 
1.5‰ lower compared to the LIA coral. Paleotemper-
ature equations based on δ18Oc in Porites indicate a 
temperature increase of 1°C for a δ18O decrease of 
0.22‰[7]. Therefore, an amplitude change of 1.2‰ 
would reflect a ~5.5 °C range in SST, or about twice 
what is observed in instrumental records. Sr/Ca ra-
tios are converted to SST suggest temperatures 
ranging between 25 and 30°C although a number of 
Sr/Ca values yield unreasonably high SSTs (FIGURE 6 
and FIGURE 7) and there is no obvious pattern to sug-
gest a seasonally related signal. High Sr/Ca ratios 
(low SST) do not correlate with low δ18Oc as would be 
predicted if changes in SST were the dominate con-
trol on both proxies. 
 

4 DISCUSSION 
The observation that changes in δ18Oc covary with 
changes in δ13Cc suggests that they are driven by a 
common mechanism— the strength of the Indian 
monsoonal rainfall. Since δ13C is not sensitive to 
changes in temperature, we conclude the most likely 
explanation is that isotopic changes in both the LIA 
and modern corals are due to changes in δ18O and 
δ13C in the local seawater during increased riverine 
discharge (freshening) resulting from the monsoon 
rains. Since δ18O in seawater averages about 0‰ and 
river as well as rain values are in the range of -5 to -
10‰, mixing these two endmembers provides a first 
order explanation to our coral results. δ18OW in the 
Bay of Bengal strongly correlates with salinity 
changes in the region, rather than purely SST[1]. From 
the equation derived from 18Ow and salinity data in 
the Bay of Bengal (EQUATION 3) a change of 1.2 ‰ in 
18Ow would represent a change in salinity of about 6 
p.s.u. (FIGURE 6), This range would likely be beyond the 
tolerance limits for a coral and its symbionts. 

We cannot, however, discount that some of 
the seasonal variability in δ18Oc is due to changes in 
SST, as suggested by the Sr/Ca data. We therefore 
conclude that about 50% of the 1.2‰ amplitude 
change in δ18Oc is due to seasonal changes in SST 
(3°C) and 50% due to salinity changes of about 3 
p.s.u.— the effects are additive. Higher SSTs during 
summer warming serve to lower δ18Oc due to a de-
crease in isotopic fraction with increasing tempera-
tures. Increased precipitation and riverine input dur-
ing the summer monsoon both add water with lower 
δ18O and thus serves to lower δ18Ow and hence δ18Oc. 
Lower δ13C is associated with lower δ18O and may in-
dicate increased riverine input of low δ13C in total 
dissolved inorganic carbon (DIC). Seasonal variation 
of around 2‰ in δ13Cc may also reflect changes in 
coral metabolism related to feeding strategy[2]. For 
example, during the winter months cloud cover is  

ABOVE: 
TABLE 1: Mean and standard deviation of the Fossil coral 
isotope values 

BELOW: 
TABLE 2: Mean and standard deviation of the Living coral 
isotope values 

 

EQUATION 3: δ18Oseawater (‰) = 0.18 × SSS(p.s.u) - 5.9(‰/p.s.u)  
P.S.U – practical salinity unit 

(3) 



  ARESTY  RUTGERS UNDERGRADUATE RESEARCH JOURNAL, VOLUME I, ISSUE II 
 
 
 

reduced and photosynthesis enhanced by the sym-
biotic zooxanthellae. During photosynthesis, 12C is 
favored over 13C so the pool of DIC becomes en-
riched in 13C thereby increasing δ13Cc during 
warmer, sunny months. During the summer mon-
soon cloud cover is increased and photosynthesis 
reduced. Corals may rely more on heterotrophic 
feeding of zooplankton and δ13Cc reflects incorpora-
tion of a pool of low 13C (e.g. -25‰). The offset be-
tween 13Cc in the modern versus the LIA coral is sig-
nificant (1.3‰) and may reflect changes brought 
about by increased and/or changing agriculture. 
Specifically, increased rice production in this region 
during the 20th century would have added a pool of 
lower 13C (~-15‰) to the riverine total DIC pool. 
  Certain values of Sr/Ca corresponding to un-
realistic temperatures may be due to local effects, 
such as changes in the Sr/Ca delivered to the study 
area by rivers or perhaps due to changes in the rates 
of calcification in the coral.  
 

5 CONCLUSIONS 
Stable isotope records obtained in both LIA 

and modern corals at St. Martin’s Island, Bangladesh 
display changes at monthly resolution, driven by the 
seasonal monsoon. We estimate variability in 18Oc is 
split equally between changes in SST and changes in 
18Ow. The average δ18Oc in the D09-01 fossil coral is 
only 0.2‰ higher compared to the living and leads 

TOP: 
FIGURE 6: Sr/Ca isotope ratios from LIA sample (Transect 3). 
Several values yield extreme values in Sr/Ca (e.g. 7 to 8) 

MIDDLE: 
FIGURE 7: Temperatures derived from the Sr/Ca ratios using 
equation 2 in the LIA coral (Transect 3).  
NOTE: Very low Sr/Ca (FIGURE 4) translate to high and ex-
treme and unreasonable estimates of SSTs (e.g. > 32°C). 

BOTTOM: 
FIGURE 8: Note the linear and positive correlation between 
δ18O of seawater with salinity, obtained from the Bay of 
Bengal. The slope suggests a change of about 0.2‰ per 
one unit change in salinity[1]. 

 



  ARESTY  RUTGERS UNDERGRADUATE RESEARCH JOURNAL, VOLUME I, ISSUE II 
 
 
 

us to conclude the monsoons were only slightly 
weaker during the Little Ice Age. That is, a combina-
tion of lower SSTs and/or increased salinity (de-
creased freshening) could explain the differences 
between the Living and LIA coral δ18O records. 
Longer records in both 18Oc and Sr/Ca will be 
needed to make a statistically significant comparison 
between the two climate states. Our pilot study 
shows the potential for generating century-long his-
torical records of the monsoon, via records of stable 
isotopes and trace metals. This would provide much 
needed spatial and temporal resolution for climate 
models and climate forecasting. Indeed, monsoons 
arise in any location where a strong land-sea contrast 
is present, from South Asia and northern Australia to 
West Africa and southwestern North America—bring-
ing to light how paleoclimate research on the mon-
soons would better quantify as well as benefit our 
understanding of the world’s climate and its impact. 
Further work should include obtaining seawater 
samples for measurements of δ18O, salinity, Sr/Ca, 
and Sr-isotope data at St. Martin’s Island along with 
measurements of trace metal concentrations in 
Porites corals that might serve as a “fingerprint” for 
identifying riverine discharge∎ 
 

6 ACKNOWLEDGEMENTS 
I would like to express my gratitude to advisor,  
Dr. Mortlock for all his support and encouragement. 
Thank you to the Aresty Research Center; Dr. James 
Wright; reference librarian, Maria Ortiz-Myers; Dr. 
Kaixuan Bu; Jennifer Nemes, Jenn Pereira, Chloe 
and the team of expert techs of the Radiology Dept. 
in RWJUH; and Mark Yu for all your help as well. 
Samples provided through funding from NSF grants 
to Dr. McHugh: OISE 09-68354, ONR N00014-11-1- 
0683. Sr/ Ca analyses were funded by Aresty Fellow-
ship Award (2018- 2019). 
 

7 REFERENCES 
[1] Delaygue, G., Bard, E., Rollion, C., Jouzel, J., Stiévenard, M., 

Duplessy, J.-C., and Ganssen, G., 2001, Oxygen isotope/sa-
linity relationship in the northern Indian Ocean: Journal of 
Geophysical Research: Oceans, v. 106, no. C3, p. 4565-4574.  

[2] Grottoli, A. G., and Wellington, G. M., 1999, Effect of light 
and zooplankton on skeletal δ13C values in the eastern 

Pacific corals Pavona clavus and Pavona gigantea: Coral 
Reefs, v.18, no. 1, p. 29-41. 

[3] Isotope Tracers in Catchment Hydrology (1998), C. Kendall 
and J.J. McDonnell (Eds.). Elsevier Science B.V., Amsterdam. 
pp. 51-86. 

[4] Mondal, D. R., McHugh, C. M., Mortlock, R. A., Steckler, M. 
S., Mustaque, S., and Akhter, S. H., 2018, Microatolls docu-
ment the 1762 and prior earthquakes along the southeast 
coast of Bangladesh: Tectonophysics, v. 745, p. 196-213. 

[5] Ramos, R. D., Goodkin, N. F., Siringan, F. P., and Hughen, K. 
A., 2017, Diploastrea heliopore Sr/Ca and δ18O records 
from northeast Luzon, Philippines: An assessment of inter-
species coral proxy calibrations and climate controls of sea 
surface temperature and salinity: Paleoceanography, v. 32, 
no. 4, p. 424-438. 

[6] Schrag, D. P., 1999, Rapid analysis of high-precision Sr/Ca 
ratios in corals and other marine carbonates: Paleoceanog-
raphy, v. 14, no. 2, p. 97-102. 

[7] Weber, J. N., and Woodhead, P. M. J., 1972, Temperature 
dependence of oxygen-18 concentration in reef coral car-
bonates: Journal of Geophysical Research, v. 77, no. 3, p. 
463-473. 


