Calibrating hydroclimate proxies in a southern Portuguese speleothem

Indroduction

Stable isotopes archived in calcium carbonate cave deposits such as stalagmites have been widely used to reconstruct past climates. Varous climatic porcesses impact the isotope ratios in both precipitation and clacite precipitation. However, various processes can influence the isotope signal recorded in speleothems, potentially obscuring climate signals. Stalagmite oxygen isotopes can record the d18O values of regional precipitation or temperature and hence hydroclimate, but processes such as soil and karst water evaporation, mixing, and seasonally biased carbonate precipitation may complicate the the isotopic signature of stalagmite. Stalagmite carbon isotopes can record soil metabolism, which is also related to hydroclimate, but are also impacted by kinetic effects during degassing and calcite precipitation. The goal of this study if to use high-resolution oxygen and carbon isotopes from in a small stalagmite (5 cm in height) with a modern top from Companheira cave in in southern Portugal. Prior anaylysis determined that the oxygen isotope ratios of the calcite indicate that the stalagmite precipitated in equilibrium with cave drip water. Equilibrium between calcite and drip water allows for the assumption that the isotopic signature of the stalagmite accurately reflects the isotopic signature of local precipitation. Given today’s modern rapid climate change, understanding how a particular climate system has changed over time and analyzing why this change may have occured is crucial. Comprehensive paleoclimate studies can be conducted once a calibration between speolothem and precipitaton is made, which is why this particular project is necessary. Following paleoclimate studies may be able to give us an idea of how our current climate change may impact regional preciptation and climate systems. Further development of time series developed in this study and a better age model will allow for a more comprehensive evaluation of dynamical changes in the regional climate system through time, particularly in the spatial extent and intensity of the Azores High, a system which strongly impacts precipitation in this region.

Regional Conext of Study Site

library(readxl)
library(tidyverse)

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Map of Study site

Portugal if located in the Iberian Penisula in the Mediterranean region. The study site is located on the southern coast of Portugal. Rainfall in Iberia is strongly related to the strength of the Azores high pressure system, and the cave site on the southern end lies at its epicenter, making it an ideal location to study paleoclimate in the Iberian Penisula.

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Stalagmite Isotopic Signature and Age Model

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Figure 1

Carbon isotope composituon of the modern stalagmite. Distance drilled into the stalagmite was correlated to a sprefic amount of years BP (1.4 years every 24 um drilled). Assuming carbon isotopes in this stalagmite responds to climatic conditions in a similar way to northern portugese spelothems, a general increasing trend of d13C values suggest drying since 1950. However carbon isotopes are particulary sensitive to isotope fractionation by the process of prior calcite precipitaton (PCP) in the overlying karst. PCP involves the degasing of CO2 in waters as they peroclate through the epikarst and cause stalagmites to record a higher d13C value than was actually in the rain water. Given the overall increaing trend overtime with now significant changes it is likely this process is impacting the d13C values of the stalagimite. Therefore d13C will not be further discussed in this study.

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Figure 2

Oxygen isotope composituon of the modern stalagmite. Distance drilled into the stalagmite was correlated to a sprefic amount of years BP (1.4 years every 24 um drilled). Assuming precipitation amount is controling isotopes signature of the rainwater, which is what has been determined as the main isotope control in northern Portugal,this stalagmite records past hydroclimate. Decreases in d18O indicate time periods of greater annual precipitation, while increases in d18O indicate time periods of drier climatic conditions. There appears to be three time periods of climatic change.

Precipitation amount influences on d18O

Determining usable historic precipitation data

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Table 1

Table of total precipitation for all of Portugal from 1901 - 2015

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Table 2

Table of two different precipitation sets, one from Faro, Portugal located in close proximity to the study site, and one representing countrywide precipitation.

Figure 3

Total annual precipitation for all of Portugal compared with regional data from Faro, Portugal (1978-2001) . Correlation suggests that the historical portugal precipitaion values are representitive of precipitation experienced in southern portugal. However, it is not a perfect relationship, which should be noted in later sections.

Figure 4

Figure to deermine which location recieves more precipitation annualy. It appears by the distribution of the box and whister plot that Portugal as a whole recieves more precipitation that Faro, which is to be expected for a specific location when compared to a regional data set. However variation is not so significant that the historic precipitation data cannot be used.

Regional Precipitation Characteristics

Yearly Historical Precipitation Trends

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Figure 5

Portugal historical precipitation data from 1901 - 2015. Data shows highly variable year to year precipitation totals with no obvious annual trends overtime.

Monthly Precipitation Trends

Table 3

Average monthly precipitation from Faro, Portugal. Local data was used because the long term historical data is only needed when assess the long record of stalagmite d18O values.

Figure 6

Bar chart of monthly precipitation averages in Faro, Portugal from 1978 - 2001. Obvious seasonal trends can be observed. Summer months are extremley dry, while winter and colder months are corrlated to a high amount of precipitation. December has the highest average precipitation while August has the lowest average precipitaton.

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Figure 7

Monthly total precipitation in Faro Portugal. Bar and whisker graph allows for the assessment of variablity in a given month. Wetter months appear to have the greatest degree of precipitaton variability, while drier months are consistently dry.

Figure 8

Raster plot made to determine if seasonality has changed over the years in portugal from 1901 - 2015. The red line, indicating the driest months has no curved up of down over the years which means that seasonlity has not shiften in portugal over the last 100 years, meaning, for example that the driest months were never october and september, they have consitently been July and August. This assure that monthly precipitation average values can be used.

Monthly d18O preciptation trends

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Figure 9

Average monthly d18O signatureof rain water. Appears to follow precipitation trend, with higher d18O values during the direst months.

#### Figure 10 Comparison of monthly average preciptation and monthly average d18O of precipitation. Precipitation amount varies greatly throughout the year while d18O remains relativley contant with no flucutions similar to precipitation amount. Howver secodary axis is not at the opitmal scale for comparison. Not convinving graphic evidnece in a relationship between these two variables.

d18O stalagmite relationship to precipitation

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Figure 11

Graph depicting the correlation of annual precipitation amount in Portugal to d18O of the stalgamite from 1900 - 2015. There is a very low correlation between these two variables. This suggests that precipitaion amount is not the main driver of precipitation d18O values as previously thought.

Precipitation amount influences on d18O

yearly historic temperature trends

Table 5

Average yearly portugal temperature from 1901 - 2015

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Figure 12

Portugal historical temperature data from 1901 - 2015. Data shows relativley rapid increasing temperature in portugal in the past 100 years. Studies have show that the Iberian Penisula is particularly sensitive to global warm and is as a result experiencing a greater amount of wild fires annually. This temperature graph supports the idea that Portugal is experiencing a significant degree of anthropogenically caused global warming.

Regional Temperature Characteristics

Monthly temperature trends

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Table 6

Table compiling the monthly average temperature of portugal and d18O average of precipitation.

Figure 13

Bar chart of monthly temperature averages in Faro, Portugal from 1978 - 2001. Obvious seasonal trends can be observed. Summer months are warmer, while winter months are colder. Howver there is not a significant amount of variablity in monthly temperature.January is the coldest while July and Augest are the warmest months. It is important to note that driest months correlate to the warmest months.

Figure 14

Comparison of monthly average temperature and monthly average d18O of precipitation. Temperature varies throughout the year and the d18O chroughout the year appears to track the temperature change. This is not a confirmation thattemperature controls d18O of precipitation, however it is an idicator of a relationship.

Stalagmite d18O and yearlt temperature

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Table 7

Tabel compieling the isotope data of the stalagmite with annaul temperature averages from 1901 - 2015.

Figure 15

Figure to show the distribution of average temperature comapred with d18O values. Temperature is distrubed on the lower end of the average range, while d18O values are also distributed on the lower end of the range. This indicates that portugal generally sees temperatures around 14˚C and d18O values may refect this.

Figure 16

Graph depicting the correlation of average annual temperature in Portugal to d18O of the stalgamite from 1900 - 2015. There is a high correlation between these two variables. This suggests that temperature of the region is the main driver of precipitation d18O values. This is a surprising discovery given that past studies have shown that d18O of precipitation in northern portugal is driven largley by precipitation amount not temperature. A reason for this difference could be that southern portugal is located closer to the cpast and experiences oneverage less precipitation. Difference in climatic systems in northern and southern portugal may acount for different d18O precipitaion influences.

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Figure 16

Ussing the understanding that temperature is the main control of stalagmite d18O values three time periods of temperature chnage can be determined by the d18O stalagmite signature. From 1800 - 1890 d18O values do not show a high degree of variability, indicating that temperatures remained relativly consistent in the region marking this as time period of climate stability. From 1890 to 1865 temperatures began to decrease, indicated by a decrease in d18O stalagmite values. However after 1965 temperatures began to increase until modern times. Based on d18O values it appears portugal modern tempertures are relativley similar to temperature experienced during the 19th centurary.

Conclusions

Precipitation d18O valyes have a strong temperature effect and weak ‘amount’ effect. This is in contrast to wetter northern Portugal where precipitation amount is the dominant control on precipitation d18O values. Three time periods of change in the southern region of portugal can be observed in the 𝛿18O record. High 𝛿18O values from 1780 to 1890 indicate regionally stable and warmer conditions during this time interval. The region began to experience general colder and more variable climatic conditions after 1890, represented by a decrease in 𝛿18O values. These colder climatic conditions ended in 1960 when temperatures gradually increased until the present. Temperature data obtained from weather stations support this isotopic trend and indicate that a higher average annual temperature occurred throughout most of 20th century. Carbon data obtained from the stalagmite suggest that carbon isotope fractionation is being driven by factors other than just soil metabolism. However, this study does support the idea that speleothem 𝛿18O of calcite accurately reflects temperature The Mediterranean region has been identified as a climate change hot spot. Recent observations, including increased fire activity, indicate that climate in the Iberian Peninsula is becoming drier and warmer. Understanding what controls isotope signature of precipitation in this region, as this study has tried to do, will allow for a better undertanding of future paleoclimate studies on stalagmites in this region.

Citations

http://www.stat.columbia.edu/~tzheng/files/Rcolor.pdf NSF project proposal thatcher, gilikin, wanamaker https://github.com/rstudio/leaflet/issues/486 https://www.sciencedirect.com/science/article/pii/S0277379115300111