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.). 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