Estimated Puerto Rican Rainfall Variability through the Little Ice Age and its implication for sustained long term shifts in the ITCZ

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1 Estimated Puerto Rican Rainfall Variability through the Little Ice Age and its implication for sustained long term shifts in the ITCZ Abstract: Meghan Gray Senior Thesis, USC Earth Sciences, Spring 2007 Advised by Prof. Lowell Stott By measuring the oxygen isotopes in a speleothem (PRD-1) taken from Perdida Cave in Puerto Rico at a subdecadal scale (18ºN 67ºW) and rainfall from Barbados, I was able to reconstruct estimates of annual rainfall variations spanning the time period between from the beginning of the Little Ice Age to the present day. The location where the Puerto Rican speleothem was taken represents the Northern boundary of the present-day Intertropical Convergence Zone (ITCZ), the belt of low pressure around the equator that controls the wet and dry seasons in the tropics. The position of the ITCZ shifts seasonally in response to changing sea surface temperatures (SST s) in the Northern and Southern Hemispheres. Long-term shifts in the ITCZ can result in severe droughts or flooding in the tropics. By analyzing the speleothem and rainfall oxygen isotopes from Puerto Rico and Barbados, respectively, insight can be gained about how the hydrological cycle functioned in the past. Periods of anomalously dry or wet conditions indicated by these oxygen isotope results may be interpreted as representing a bias in the seasonal extent of the ITCZ s migration. During the Little Ice Age the northern tropics were cooler and it is hypothesized that this would have been associated with a southerly bias in the northern extent of the summer-time ITCZ. Under such conditions the average, summer-weighted precipitation falling on Puerto Rico would have been lower than today. We observe a progressive increase in the δ 18 O values in the PDR-1 spelethem calciate record between 1400 and ~1550AD, brief return to lower δ 18 O values centered between 1670 and 1770AD, then another interval of higher δ 18 O values between 1780AD and first two decades of the 20 th century. By converting these values to a predicted rainfall history we conclude that the LIA was indeed characterized by three distinct multi-decadal periods of reduced rainfall, consistent cooler SSTs in the northern tropics. Understanding the behavior of the ITCZ and the mechanisms responsible for its shifts will help predict future climate shifts in this region and the effects it would have on the populations living in these tropical areas. INTRODUCTION The ITCZ is belt of low pressure around the equator caused by the evaporation of warm, moist air in the tropical latitudes (Fig. 1). The ITCZ controls the hydrologic cycle in tropical regions, and the populations in these regions depend upon the rain it brings to irrigate their crops thus sustain their livelihoods.

2 The ITCZ s position shifts annually in response to seasonal heating and cooling of the surface ocean in the Northern and Southern Hemispheres. It will shift towards the Hemisphere that has warmer SST s. Thus, during the summer months of the Northern Hemisphere, the ITCZ shifts northward, bringing moisture to the northern tropical latitudes and leaving the southern tropical latitudes drier. The opposite occurs during the summer months of the Southern Hemisphere. This migration of the ITCZ creates the annual wet and dry seasons that occur in the tropics. If one of these Hemispheres maintains SST s that are considerably lower or higher than the other for a prolonged period of time, the ITCZ will shift north or south for that period of time, causing it to be drier where the ITCZ no longer reaches. The Little Ice (LIA) is a period marked by relatively cooler conditions in the northern tropical latitudes (reference) and this might be expected to have caused a southward shift in the summertime latitude of the ITCZ. Fig 1. Present-day location of the ITCZ during July(red) and January (blue). The LIA was a period of time from the beginning of the 15 th century to the middle of the 19 th century when the Northern Hemisphere cooled by less than 1 C (Bradley and Jones, 1993; Hughes and Diaz, 1994; Crowley and Lowery, 2000). This cooling of the Northern Hemisphere should have caused a southward shift in the ITCZ, which would have resulted in drier conditions in Perto Rico where PDR-1 was collected. Oxygen isotopes from the speleothem PDR-1 were used to access whether or not this shift in the ITCZ took place during the LIA. By measuring the ratio of 18O to 16O (d18o) against a standard (d18ovpdb) a general idea can be gained as to when wetter and drier periods occurred. I have interpreted the d18o variations in the PDR-1 speleothem to reflect the amount of rain that has fallen on the Perdida Cave. As a cloud travels from its source region to a given location (i.e., Perdida Cave), it looses the d18o-enriched water vapor because it is heavier. It stands to reason that at the source, there will be more rain and thus a greater amount of d18oenriched water and as the cloud moves away from the source there will be less rain and thus less do18-enriched water. For this reason, I interpreted the more negative isotopic values to indicate wetter conditions and the less negative or positive values to indicate drier conditions. There are several natural processes other than rainfall that can contribute the variations in speleothem d18o, such as changes in evaporation in the soil zone, humidity within the cave, groundwater

3 infiltration rate, and varying drip rates (Sinha et al., 2005). However, all these processes force the oxygen isotope ratios in the speleothem calcite in the same direction as the aforementioned amount effect, that is negative isotope values indicate wetter conditions (Burns et al., 2002, Fleitmann et al., 2003). Because the d18o alone only gives us a relative idea of how dry or wet the conditions were, I tried to assign actual rainfall amounts that correlate to the d18o in the speleothem calcite that spans from the present to the beginning of the LIA. METHODS Three speleothems were collected from deep within Perdida Cave in Puerto Rico. PDR-1 was one of two speleothems collected from under an active drip, and thus was presumed to be growing when it was collected. There is a river that runs through the cave, which contributes to the high relative humidity in the cave throughout the year. Two data loggers were also placed in the cave to monitor temperature and humidity. The temperature has remained at a constant 22 C, since it has been measured. Samples where taken from the top and bottom and were dated using the U/Th technique. (Winter et al., 2007) There were 4 age dates that were taken to develop an age model. The measurement at the top was taken at 3mm depth and was dated at 61 +/-12 years bpc(??), and the bottom was taken at 148 mm depth and was dated at 10,448 +/- 91 years bpc (??). The growth axis and the age model of the speleothem indicated continuous growth with few if any hiatuses. The 230-Th age dates indicate that PDR-1 grew continuously through the LIA. Calcite samples from the speleothem were drilled at 20 micron intervals along the growth axis with a microdrill, which represent approximately one year of growth and thus one year of d18o and rainfall. A total of 617 samples were taken. The first 130 samples of PDR-1 were analyzed for the d18o. After sample 130, every fifth sample was analyzed for the d18o due to time constraints. All samples were analyzed with an online automated carbonate preparation system linked to a VG Prism II ratio mass spectrometer by means of standard preparation techniques (Sinha et al., 2005). The d18o values with respect to Vienna peedee belemnite (VPDB) by way of intercalibration with National Institute of Standards (NIST) carbonate standards ( Sinha et al, 2005). I obtained a rainfall record of Puerto Rico from Amos Winter, which we used in conjunction with a record of rain d18o values from Barbados to develop a regression line that could be used interpolate the rainfall amounts through the LIA. The Barbados d18o record has monthly rainwater d18o values for the year The Rainfall record from Puerto Rico inlcudes monthly rainfall amounts in millimeters from I seperated out the monthly rainfall amounts and rainwater d18o for each year. For the rainfall I took the monthly averages over the 98-year period. For the rainwater d18o, I did the same thing and took the average over the 20-year period. I then plotted these average values against each other to obtain the following regression line and its equation (Fig. 2).

4 Average Monthly Rainfall y = x R 2 = Average Monthly d18o-sm Fig 2. Plot of average monthly rainfall from Puerto Rico and d18o from Barbados rainfall. In the equation y = x , y = the amount of rainfall and x = the d18o-smow value. The d18o in PDR-1 is from the speleothem calcite, so in order to predict rainfall amount from these values I estimated how the speleothem calcite d18o values should change in response to varying rainfall over the cave location. Today the oxygen isotope value of monthly rainfall varies systematically with rainfall amount, the so-called amount effect. The relationship between average monthly rainfall over Puerto Rico versus the d18o of average monthly rainfall over Barbadoes (a closest location to the cave for which there is a d18o data base) is shown in Figure 3a. asonal rainfall is stronconverted the d18o-smow values into d18o-pdb values using the following equation: t( C) = (d18O-PDB d18o-smow) (d18O-PDB d18o-smow) 2 (Eqn.1) Because the temperature remained at a constant 22 C in the cave, I was able to solve for d18o- PDB. Using these values instead of the d18o-smow yielded nearly the same results (Fig. 3). Using these new d18o-pdb values I developed a new regression line that more accurately interpolates the rainfall amount through the LIA that fell on Puerto Rico at this location.

5 Figure 3a. Averaged monthly rainfall and its δ 18 O value from Barbados for the years, Average Monthly Rainfall Predicted Calcite! 18 O vs Rainfa y = x R? = Average Monthly Rainfall -VPDB d18 Fig 3b. Average monthly rainfall from Puerto Rico and δ 18 O- VPDB calculated using Eqn 1. I then used the regression line equation from Fig. 3b to calculate average monthly (summerweighted) rainfall back through the LIA using the δ 18 O values obtained from the analysis of the PDR-1 speleothem calcite (Figure 4).

6 Fig 4. Estimated average monthly (summer weighted) rainfall using the predicted calcite δ 18 O vs. Rainfall Regression Equation (Fig. 3b). RESULTS AND DISCUSSION: The graph in Fig. 4 depicts multi-decadal fluctuations in rainfall over Puerto Rico from the present day back through the LIA. The rainfall values range from 95 mm in 1905 to 193mm in These values are more heavily waited towards the summer time rainfall values since most of the rain falls during the summer months in this region (Fig. 5). During the beginning of the LIA, rainfall amounts are at their highest amounts in the time interval that was analyzed ( ), but they gradually decrease from 188mm in 1475 to128mm in This agrees with the hypothesis that as it got colder during the LIA, the rainfall amount in Puerto Rico would have decreased due to a southward shift in the ITCZ. From 1696 to 1754 the rainfall amount increases from 128mm to 183mm, respectively. Following the mechanisms of the previous hypothesis, this would mean there must have been some warming during the LIA from approximately 1770 to 1760, which allowed the ITCZ to migrate further north to provide rain for locations such as Puerto Rico along the northern border of the ITCZ. From 1754 the rainfall amount decreases gradually to 95mm in 1905, which is the lowest rainfall recorded in this record. It is interesting that the lowest predicted rainfall amount was not during the LIA. The rainfall amount increases to 172mm in 1954, and then decreases again to 123mm in It increased for its final time until 1986 with 153mm of rainfall, and then has been steadily decreasing since 1986 to the present. In total, there are 3 major signals where rainfall decreases and then increases, but each of the signals get progressively smaller with time. The biggest signal being during the LIA makes

7 senses, because that it when presumably the ITCZ had shifted southward. The mechanisms that could explain the increases and decreases in the rainfall post LIA are uncertain. Average Monthly Rainfall Rainfall (mm) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Month Fig 5. Average Monthly Rainfall from Puerto Rico There is evidence that this southward shift in the ITCZ zone was not a localized occurrence in the Caribbean. Newton, Thunell, and Stott presented evidence in their 2006 paper, that there was also a southward shift in the ITCZ zone in the Pacific Ocean (2006). They used planktonic foraminiferal Mg/Ca and d18o to derive SST s and salinity records from the Makassar Strait near Indonesia. Their results showed the coolest temperatures and lowest salinities occurring during the LIA (Fig. 6). This suggests an overall a southern displacement of the ITCZ during the LIA causing there to be drier conditions in the northern tropics and wetter conditions in the southern tropics. (Newton et al., 2006)

8 Fig 6. (a) depicts the derived temperatures estimates from Mg/Ca from Globigerinoides rubber (a specific species of planktonic foraminifera). (b) Meaured d18o of calcite in the shells of Globigerinoides rubber. (c) Mg/Ca and d18o derived from seawater d18o. (d) reconstructed sea surface salinities. (Newton et al., 2006) This evidence supports my Puerto Rico speleothem results, suggesting that there was a global southward displacement of the ITCZ during the LIA, and it was not a localized event. This holds importance in trying to predict future rainfall patterns for agricultural purposes in the Caribbean as well as other regions affected by the ITCZ migration. It is interesting that with the global warming that has been occurring since 1850, that the ITCZ has not been shifted significantly north or south. However, this is probably due to the fact that both hemispheres are warming evenly, so one hemisphere is not getting considerably warmer than the other like during the LIA. One area of concern for the ITCZ involving global warming is the effect that it will have on Thermohaline Circulation in the North Atlantic. As the glaciers are melted on Greenland, large amounts of freshwater are being added to the North Atlantic. This freshwater would prevent the warm water from the equator from sinking, thus preventing this warm water from reaching the northern latitudes. The Northern Hemisphere would experience another cooling similar to the LIA, causing the ITCZ to shift southward yet again. However, the impact of the resulting droughts would be much more catastrophic because there are more people that live in the northern tropics now than during the LIA that would be adversely effected, and the world as a whole has a heavy dependence upon produce coming from these tropical regions.

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