Past present and future: Change of land surface area of Habitats on Crete since 18 ka

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Past present and future: Change of land

surface area of Habitats on Crete

since 18 ka

Bachelor Thesis Lieve Denkers 11346760 5263 Words 4-8-2020, Amsterdam Supervisors: dr. K.F. (Kenneth) Rijsdijk dr. A.C. (Harry) Seijmonsbergen

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Abstract

Since the Last Glacial Maximum (LGM), melting of ice sheets caused an eustatic sea-level rise of about 135 meter. Together with tectonic activity, hydro- and glacio-isostasy and a changing geoid this late Pleistocene and Holocene sea-level rise changed land surface areas. In the Aegean region, where Crete is located, the sea level rise caused a decrease in land surface area of approximately 70%. The area that is submerged under sea-level was in many cases a coastal plain. These coastal plains could have been suitable habitats for larger mammals, such as humans and elephants. The change in the relative sea level of Crete is mapped to see to what extent the tectonic activity around Crete has influenced the relative sea level. Subsequently the change of habitable surface area for humans and elephants since the last glacial maximum is calculated. Moreover, the contribution of the vertical tectonic movements of Crete to the relative sea level change is calculated These results have been compared to previous research where the tectonic component was neglected. Thereafter, with the criteria of an elevation below 700 m and a slope of 3° or less, the most suitable areas for elephants and early humans were selected and the change in suitable surface area has been calculated. The resulting surface area of Crete since 18 ka have been graphically depicted in 19 maps. About 20% of the total land surface area of Crete has been submerged since the LGM and this submerged area contained 39% of the habitable area of Crete. The tectonic movement of Crete causes an increase in surface area since 6 ka. Using this method to find the rate of change in the past could be used to make a prediction of the future under the predictions of IPCC projected global sea-level rise. The change in sea level rise especially has a strong effect on the habitable areas, therefore the results of this research could be used as a model for future impacts of sea-level rise on mammals living in coastal zones.

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1. Introduction

As predicted by the International Panel of Climate Change (IPCC) the sea level will rise almost 1 m by 2100 due to human induced climate change (IPCC, 2013). This sea level rise may cause flooding of coastal areas and thus a change in the land surface area of coastal regions. Ecosystems might be affected by the reduction of their land surface area, resulting in shifts in immigration and extinction rates, genetic dilution and geographic speciation (Fernández‐Palacios et al., 2016). Humans have been confronted with rapidly changing sea levels before. About 20.000 year ago (20 ka) during maximum glaciation of the last ice age, the global sea level reached

its lowest point. When this last glacial maximum (LGM) ended about 18 ka, the relative sea level in the Mediterranean Sea started to rise. Since then the sea level has risen up to 135 m causing flooding of larger areas of land (Fleming et al., 1998; Lambeck, 1995; Lambeck, 1996; Marshak, 2015; Tiberti et al., 2014).

Crete is an island in the Aegean region in the north eastern Mediterranean Sea (Figure 1). Crete is a mountainous island with a total surface area of 8265 km2 _{(Lazarina et al., 2019). Three mountain}

massifs can be distinguished. Levka Ori, or white mountains, are located in the west with an elevation of 2453 m. Psiloritis in central Crete has the highest altitude with an elevation of 2456 m. To the east Dikti can be found with an elevation of 2148 m (Bergmeier, 2002; Lazarina et al., 2019). Along with the sea level changes from the melting of the ice sheets this region has been subjected to tectonic uplift during the Pleistocene and Holocene. This tectonic uplift is caused by the subduction at the Hellenic trench to the south of the Aegean region (Lambeck, 1995; Mouslopoulou et al., 2017).

1.1 Mammals and their habitat on Crete

In the Miocene 9 million year ago (9 Ma) Crete was connected to the mainland, until 5 million years later when Crete completely submerged under the sea in the Pliocene. Subsequently it re-emerged from the sea about 1,8 Ma. The sequence of being connected to the mainland, submerging and re-emerging as an island makes Crete an Ocean-like island (Fernández-Palacios et al., 2016; Van der Geer et al., 2006). Colonization of oceanic and ocean-like islands by new species of mammals can happen by swimming, floating or rafting (Blackburn et al., 2004; Van der Geer et al., 2006). Because of the occasional arrival of newcomers on such islands, they often harbour a low species diversity. Moreover, the species that do arrive on these islands often evolve into dwarf or giant versions of their mainland ancestors (Fernández‐Palacios et al., 2016; Masseti 2012; Van der Geer et al., 2006).

In the Pleistocene before human arrival two different paleo biota with slightly different animal species lived on Crete. It is unknown why the biota of the first biozone became extinct during the middle Pleistocene and Crete got colonized by new species in the so called Mus-biozone (Van der Geer et al., 2006). At the end of the LGM Crete was colonized by two species of mice, Cretan shrew, dwarf elephants, eight different sized Cretan deer and the Cretan otter (Van der Geer et al., 2006). Around 10 ka the animals of the Mus-biozone went extinct just before or closely after the arrival of humans (ibid.).

Kapsimalis et al. (2009) state that the now submerged low shelf areas in the Aegean region, with an average slope of 1.58°, might have provided the most suitable areas for humans, animals and plants. With the arrival of humans and the rise of the sea level these suitable areas might have decreased in

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5 size while they increased in population density. The first evidence of human arrival on Crete are engravings dated back to upper Palaeolithic times, at least 11 ka (Strasser et al., 2018). Evidence of human settlement in Crete is dated back to 10-9 ka in the form of agricultural remains (Broodbank et al., 1991). Most of the remains of early human settlements in Crete have been found below 800 meters (Driesen, 2001). The lowlands (<700 m) of Greece were regarded as most suitable area for agriculture and settlements in Neolithic times (Demoule et al., 1993; Kapsimalis et al., 2009). If and how humans were the reason of the extinction of the late Pleistocene Mus-biozone is unknown (Van der Geer et al., 2006). The decrease in land surface area in the Aegean region caused by sea level rise was proposed to be a cause for more local extinctions, increased genetic bottlenecks and higher genetic diversity in species pools by Simaikis et al. (2017).

1.2 Relative sea level change

The melting of ice sheets will have a different effect on nearby areas in comparison to far field areas. Due to the variety of processes, the relative sea level rise can vary from location to location (Figure 2) (Khan et al., 2015; Lambeck, 1996; Marshak, 2015). Since this research is focused on the area around Crete, the processes influencing this area have been set out in more detail. The relative change of the sea level at a specific location can be schematically expressed by Equation 1 (Lambeck et al., 2005).

∆𝜁𝑟𝑠𝑙(𝜑, 𝑡) = ∆𝜁𝑒𝑠𝑙(𝑡) + ∆𝜁𝐼(𝜑, 𝑡) + ∆𝜁𝑇(𝜑, 𝑡) (Equation 1)

∆𝜁𝑟𝑠𝑙(𝜑, 𝑡) represents the change of the sea level at a certain location relative to the land over a certain period of time. The first part of the equation, ∆𝜁𝑒𝑠𝑙(𝑡), represents the change of the ocean volume. The second part, ∆𝜁𝐼(𝜑, 𝑡), represents the isostatic changes. The last term ∆𝜁𝑇(𝜑, 𝑡) represents the tectonic component of the relative sea level. The

contribution of each term differs per location on earth. In the following paragraphs, these three processes will be further explained.

Increase in oceanic volume is caused by the meltwater of the melting of excess ice. Excess ice is the part of the ice on the ground that exceeds the amount of meltwater fitting in the pores of the soil underneath the ice. At the LGM the excess ice had a volume of 40 to 50 million km3_{(Milne et al., 2002; Ballantyne, 2018).}

The oceanic volume term of the relative sea level equation became increasingly dominant as the ice sheets melted (Flemming et al., 1998).

Isostatic contribution can be divided in three parts that are closely connected and influence each other. The first two parts are hydro- and glacio-isostasy, the third part regards changes in the gravitational potential of the earth.

Isostasy is the state of equilibrium of the forces between the earth’s crust and the underlying mantle. The relatively stiff crust floats on the more fluid mantle. When a larger force presses down on the mantle, the force from the mantle pressing up on the crust will equally increase. The upward force can cause uplift or subsidence of continental or oceanic crust, this process is called isostatic compensation.

Figure 2. Relative sea level at two locations in the Mediterranean sea for the isostatic and oceanic volume components of the relative sea level equation (Lambeck, 1995).

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6 The forces can have multiple causes, such as glaciers, liquid waterbodies, or mountains (Marshak, 2015; Stouthamer et al., 2015). The ice sheets in northern Europe, northern America and Antarctica during the LGM caused the crust at these locations to start sinking under the pressure of the ice. Due to isostatic compensation the crust around the ice sheets is lifted up to create a new isostatic equilibrium. When the ice sheets start melting, the pressure from the crust down decreases and the once sunken crust starts to lift up. The crust around the melting sheets, once uplifted, starts to subside forming a new isostatic equilibrium (Marshak, 2015). The effects of isostasy cause the crust around the Aegean region, including Crete, to subside. This occurs because of the decreased pressure of the ice sheets (glacio-isostasy) and the increased pressure of oceanic water volume (hydro-isostasy; Lambeck, 1995). On Crete the relative sea-level change is induced by both (hydro- and glacio-)isostasy and tectonic activity. The third part of the isostatic component interferes in the before explained hydro- and glacioisostasy by defining how the water will be distributed around the world. The water around the world is distributed over the planet in such a way that the water surface at every location has the same gravitational potential. The ice sheets in northern Europe, northern America, and Antarctica during the LGM were a great mass on top of the earth’s surface. As gravitation is the attraction or force between masses, these massive ice sheets attracted sea and ocean water, pulling it upward towards the ice. With the melting of the ice sheets the gravitational potential of the earth shifted away from the poles (Lambeck et al., 2005).

The area around Crete has been undergoing tectonic uplift due to the active Hellenic trench (Meier et al., 2007; Van Andel et al., 1982). Since about 20-15 Ma the African plate subducts under the Aegean plate, creating the Hellenic subduction zone (Figure 3). This subduction zone causes different rates of uplift over the island of Crete (Figure 4) (Lambeck, 1995; Meier et al., 2007; Strobl et al., 2014). In the W-SW part of the island the uplift rate is about 4.4 mm/y, while at the central south of the island the rate is about 0.8 mm/y and the rate in the E-SE part is about 2.0 mm/y (Lambeck, 1995, Strobl et al., 2014).

1.3 Research aim

The aim of this research is to determine to what extent the tectonic component effects the change in land surface of Crete, and how this change in land surface area has affected the habitable area for humans and elephants. In this research the change of the land surface area of Crete since the last glacial maximum is reconstructed. This reconstruction is done by calculating the vertical change of Crete and the ocean surrounding it per time step of 1 ka, resulting in 19 maps. Subsequently these maps have been divided into suitable habitat area or unsuitable habitat area for humans and Cretan elephants. Previous predictions on the past relative sea level around Crete have taken into account

Figure 3. Location of the Hellenic subduction zone, in the NE Mediterranean Sea (Strobl et al., 2014).

Figure 4. Rates of tectonic uplift on Crete in mm/y since LGM. The vertical and horizontal axis refer to the latitude respectively (Lambeck., 1995).

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7 the oceanic volume and isostatic components, resulting in a rising relative sea level as can be seen in Figure 2 (Lambeck, 1995; Koene, 2013). It is expected that with the addition of the tectonic

component to the equation the relative sea level rise in Crete slightly less. This reduced sea level rise is expected because of the direction of the tectonic movements of the earth’s crust around Crete is upward. The inlands of Crete mainly consist of mountains, while the coastal areas contain most of the lowlands (Bergmeier, 2002; Lazarina, et al., 2019). As the lowlands are seen as the most habitable area by Kapsimalis et al. (2009), it is to be expected to find greater decrease of habitable area

compared to the not habitable area.

2. Methods

2.1 Workflow diagram and data

To create paleo area maps of Crete ESRI’s ArcGIS Pro 2.4 was used. The data used in this research is shown in Table 1. First a literary research on early inhabitants of Crete and their habitat resulted into criteria for the habitat of prehistoric elephants and early humans. At the same time, a literary research has been conducted on relative sea level and the tectonic movements in the region of Crete. This resulted in a dataset, which was used to create a map of the vertical tectonic uplift of Crete, as well as 19 maps previously made by Koene (2013). These previously made maps had only taken into account the oceanic volume component and the isostatic component. From both the uplift map and the maps from Koene (2013) 19 maps depicting Crete at relative sea level since 18 ka are shown. Together with the criteria these maps resulted into 19 maps showing the habitable area of Crete since 18 ka.

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Table 1. Meta data required for conducting this research.

2.2 The three relative sea level components

Koene (2013) performed research on the relative sea level change of the past 21 ka in north eastern Mediterranean sea. This research mapped the oceanic volume term and the isostatic term of the relative sea level equation (see Equation 1). The maps from 18 ka until present from research of Koene will be used. The tectonic component of the relative sea level equation will be calculated and combined with these maps. This project focuses on the true surface area of a location in Europe, therefore the “Europe Albers Equal Area Conic” Projected coordinate system was used. In ArcGIS Pro the maps of Koene are projected to the “Europe Albers Equal Area Conic” Projected coordinate system with use of the Raster Calculator function. To ensure that all maps had the same extent, a polygon feature was created to use as clipping extent. This polygon feature is called “Area Of Interest”. Subsequently the projected maps of Koene are clipped to the extent of the Area Of Interest, with use of the Extract By Mask function.

To create a map containing the uplift rate of the area of Crete in m/1000 years, literary research is conducted. From three different studies, data has been collected on the vertical tectonic uplift rate (Figure 4; Figure 6; Figure 7). In this research a

constant vertical tectonic movement is assumed since 18 ka until present. A map with data of the tectonic uplift in Crete (Lambeck, 1995) isdigitalized in ArcGIS Pro. Georeferencing was used to set the image to the right coordinate system. Subsequently a feature layer was created and point data added. By hand all lines and points shown in Figure 4 were digitalized and the data were added to the attribute table. This resulted in 1561 data points.

Data Data type Specifications

19 Digital Elevation Models of Crete from 18 ka until present

TIF-format with 955 m spatial resolution

Koene, 2013

Mean global sea-level rise IPCC prediction

Excel work-sheet Table All.7.7 contains data on predicted mean global sea-level rise for 2007-2100. Obtained from:

http://www.ipcc-data.org/sim/index.html Vertical movements of Crete Existing literature

(1597 data points)

Ganas et al., 2009; Lambeck, 1995; Robertson et al., 2019

Active Faults KML File Ganas et al., 2013

Human and elephant habitat Criteria elevation and average slope

Existing literature Demoule et al., 1993; Kapsimalis et al., 2009

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9 Next, data was obtained from the study performed by Robertson (2019). Profile locations (Figure 6) and their data are retrieved from the paper and added as data points to the same feature layer. This resulted into 17 data points.

The third study utilized in this study is performed by Ganas (2009). Figure 7 was retrieved from this research and digitalized in ArcGIS Pro the same way as the figure from Lambeck (1995). From this research 19 data points were added to the feature layer.

The point data feature layer of the tectonic uplift of the area around Crete has been interpolated. The interpolation method used in ArcGIS Pro is the Kernel Interpolation with barriers function, with polynomial5 as function, producing first order polynomials and an output type of prediction values. The barriers used are faults in the crust of the area around Crete that demarcate specific uplift or subsidence areas (Ganas et al., 2013).

The interpolated layer was then studied and 13 more data points were added to the feature layer to increase the geologically accuracy of the output raster of tectonic uplift. In Hengl (2006) an equation is proposed to find the right pixel size for an interpolated map with N data points covering a certain area A (Equation 2).

𝑝𝑖𝑥𝑒𝑙 𝑠𝑖𝑧𝑒 = 0.0791 ∗ √𝐴

𝑁 (Equation 2)

With a total number of 1610 data points the recommended output cell size for interpolation following Equation 2 is 381 m by 381 m. With these added data points a new interpolated uplift raster was created in the same way as the previous output raster with the recommended pixel size of 381m by 381m (Figure 8).

2.3 Creating relative sea level maps

For the creation of the maps of Crete at relative sea level through the past 18.000 years up till the present, the map of the present was taken as reference map. The elevation values on the map will stay equal to the elevation as calculated in Koene (2013). Only the pixel size of the elevation map has been adjusted to be equal to the pixel size of the uplift raster. To set the projected elevation map made by Koene (2013) to the same pixel size as the uplift raster, the uplift raster has been subtracted and added to this map with use of the Raster Calculator tool. In the environment settings of the Raster Calculator tool the pixel size is

set to 381. To calculate the relative sea level for 1 ka until 18 ka the maps of Koene (2013) and the

Figure 7. Uplift rates on the map of the Aegean region (Ganas, 2009).

Figure 8 Map showing the uplift over the area of Crete in m/1000 years. The different colours show the different rates of uplift. The grey line shows the outline of Crete at 18 ka.

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10 uplift raster are combined. In the Raster Calculator tool the following calculations are executed, see Equation 3.

(𝐾𝑜𝑒𝑛𝑒𝑖 𝑘𝑎 – ( 𝑖 𝑘𝑎 ∗ uplift raster)) 𝑤𝑖𝑡ℎ 𝑖 𝑘𝑎 = 1: 18 (Equation 3)

𝐾𝑜𝑒𝑛𝑒𝑖 𝑘𝑎 represents the projected elevation maps from Koene (2013) from 1 until 18 ka.

𝑖 𝑘𝑎represents the amount of thousand years ago and the amount of times the uplift raster will be subtracted. uplift raster represents the uplift raster made in this research (Figure 8). Now raster maps of every 1000 years since 18 ka were made. All raster cells with an elevation equal to or higher than 0 where extracted using Extract by attribute tool. The output raster’s contained the cells of the area of Crete above sea level, and thus the land surface area at a certain point of time.

2.4 D

ef

ining Habitats

From the available scientific literature, criteria for suitable habitats for humans and elephants on Crete were selected. Due to time limitations the criteria for suitable area chosen in this research are simplified to only slope and elevation.

Figure 9. Elevation map of Crete 18 ka. The green area represents the area with an elevation equal to or lower than 700 m above sea level. The brown area represents the area with an elevation above 700 m.

Figure 10. Slope map of Crete 18 ka. The area’s depicted with dark or light green colour have a slope equal to or less than 3°.

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11 For the elevation criterion a maximum elevation of 700 meters above sea level was chosen to be most suitable for early human life (Figure 9). The land with an elevation of 700 m and lower are the lowlands, which probably have been the most suitable locations for agriculture (Demoule et al., 1993). Areas with an average slope of 1.58 degrees (or less) were shown to be the most suitable area for humans and animals (Kapsimalis et al., 2009). For this reason a slope of 3° or less was chosen as slope criterion. A slope map of every elevation map with cells above sea level was made using the Slope Tool function in ArcGIS Pro (Figure 10). In the symbology of these maps the classification is set to 5 classes, to the same extent as the classes of the slope map used in Kapsimalis et al. (2009).

Using the Raster Calculator tool 19 new raster layers were created from the slope raster maps and the elevation raster maps. For every step the slope map and elevation map of a certain time were used with the following command, “slope <= 3 & elevation <= 700”. This resulted in 19 maps with habitable area and non-habitable area (Figure 14; Appendices).

3. Results

3.1 Contribution of the tectonic component

The land surface area is compared to the total area of land above sea level 18 ka. The previous research by Koene (2013) calculated the relative sea level in the Aegean region with only the oceanic volume component and the isostatic component, leaving the tectonic component completely out of the equation. With the method of Koene (2013) the land surface area of Crete reaches its minimum at 6 ka with a land surface area of 77% of that at 18 ka and has stayed constant until present (Figure 11). Including the tectonic component however, this results in a land surface area that also reaches its minimum at 6 ka, but with a land surface area of 78% of that at 18 ka. After 6 ka an increase in land surface area of 2% to present is visible. Therefore with the inclusion of the tectonic component this results in a total land surface area decrease of 20% between 18 ka to present.

3.2 Change in habitable land surface area

With the method used in this research, where all three relative sea level components are added to the calculation, the total land surface area of Crete decreased from 10370 km2_{to 8342 km}2_{, this is a change}

of 20% (Figure 12). Apart from the change in total land surface area, a change is visible in the

-25% -20% -15% -10% -5% 0% 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 C ha ng e in lan d sur face ar ea (%) Time (ka)

Total surface Area change since 18 ka Denkers 2020 Surface Area change since 18 ka Koene 2013

Figure 11. Graph of percentage of change in land surface area of Crete. The blue line indicates the change of land surface area taking into account the oceanic volume change and isostatic components (Koene, 2013). The brown line indicates percentage of change in land surface area considering both oceanic volume, isostatic components and the tectonic component.

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12 proportion of habitable and not-habitable area on Crete. On the map of 18 ka (Figure 14) 32% of Crete was habitable area, while at present time this has decreased to 24% of the surface area.

For the habitable area the highest flooding rate was between 15 and 14 ka, in this period almost 10% of the land got flooded. Between 15 and 14 ka the average rate of flooding of land was 0.3 km2_/year.

In total, 39% of the habitable area has been flooded over the past 18 ka years.

Since 6 ka the total land surface area has been increasing, during which a total of 3 % increase has occurred (Figure 13; 14). The increase in land surface area has predominantly affected the habitable area, which has increased with 6 % since 6 ka.

0 2 4 6 8 10 12 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Lan d sur face ar ea (1 00 0 km ²) Time (ka)

Total surface Area (Km²) No habitat surface area (km²) Habitat Surface Area (km²)

Figure 13. This graph shows the change of surface area of Crete over the past 18 ka. The brown line indicates the total land surface area, the red line indicates the not suitable habitat area and the green line indicates the habitable surface area.

Figure 12. This graph shows the percentage of change of surface area of Crete over the past 6 ka. The brown line indicates the percentage of the total land surface area, the red line indicates the percentage of the change in not suitable habitat area and the green line indicates the percentage of the change in habitable surface area.

0% 1% 2% 3% 4% 5% 6% 7% 6 5 4 3 2 1 0 C ha ng e in lan d sur face ar ea (%) Time (ka)

Total surface Area (Km²) No habitat surface area (km²) Habitat Surface Area (km²)

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Figure 14. This figure demonstrates 4 maps of Crete showing the habitable area and not habitable area at four times, present; 6 ka; 12 ka and 18 ka. The red colour on the map indicated not habitable land surface area. The green colour on the maps show the habitable surface area of Crete. For each map the percentage of habitable area and not habitable area are shown.

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4. Discussion

4.1 Aim

In this research the aim was two-sided. The first part of the aim was to find the influence of the tectonic component to the relative sea level of Crete. Without a tectonic component a decrease in land surface area has occurred from 18 ka until 6 ka, after which no change in land surface area occurred. With addition of the tectonic components the land surface area also decreased from 18 ka until 6 ka. However, the results show an increase in land surface area since 6 ka. This would indicate a decreasing relative sea level in the past 6 ka for Crete. As expected, the influence of the tectonic component affected the relative sea level in such a way that the increase is smaller compared to the relative sea level without a tectonic component. But it was not expected that the tectonic component would be of such an influence to cause the decrease in the relative sea level as is found after 6 ka.

The second part of the aim was to determine whether the change in land surface area has affected the habitable area of Crete. The habitable area for elephants and humans was impacted most by the change of land surface area. The most rapid change in land surface area in the past 18 ka was found to be between 15-14 ka. In this period of time a great change of habitable surface area occurred, as the habitable area decreased in the period with a rate of 0.3 km2_{per year. Moreover, the habitable area}

has increased relatively the most in the past 6 ka.

4.2 Previous research

Our findings regarding the relative sea level are in line with previous work by Dominey-Howes (2015), where paleo shorelines on Crete have been dated back to 3 ka. These are old shorelines, now a few metres above sea level.

The findings on the increase of land surface area after 6 ka in this research can possibly be explained by a diminishing influence of the oceanic volume and isostatic component of the relative sea level equation. Around 9 ka the major melting of the Fennoscandian ice sheet covering northern Europe ceased and around 6 ka the melting of the Laurentide ice sheet covering a large part of northern America also came to an end (Lambeck, 1995). The ceasing of the melting of those ice sheets can have influences on both the oceanic volume component as well as on the isostatic components. When less or even no more melting occurs, the amount of water in the sea will not increase. For the oceanic volume this means the only volume increase will occur due to expansion of the oceanic water caused by warming of the oceans. At the same time, no change in the weight of the ice volume will slowly cease the isostatic movements of the earth’s crust. Therefore over time, the influences of both the oceanic volume as the isostatic components decrease after the melting of the ice sheets.

In the past 18 thousand years most of the submerged land has been a habitable area for humans and elephants. The reducing of the size of their habitat could have played a part in the extinction of the elephants and possible other animals in the Mus-biozone (Simaikis et al., 2017; Van der Geer et al., 2006). Besides the reducing of the habitat for the elephants of Crete a new species (humans) arrived at the island around the same time as their extinction (Broodback et al., 1991; Strasser et al., 2018; Van der Geer et al., 2006). Humans and elephants both prefer rather flat areas. Even though no evidence has yet been found on humans hunting on elephants, with the reducing of size of this preferred habitat the species of the Mus-biozone and the humans might have had to compete for the same habitat. This could possibly also have played a part in the extinction of the elephants on Crete. Another possibility could be new diseases brought over by early human seafarers and their livestock to Crete, causing extinction of the animals on Crete.

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4.3 This research

This research uses a more extensive method to define the change in land surface area compared to previous research, resulting in new insights regarding Crete’s paleogeology.

However, due to a technical limitation in this research the resolution of the maps and therefore the significance could be better. The DEM used from the research by Koene (2013) had a resolution of 1 km2_{. This results in somewhat squared edges of the island which can subsequently result in an offset}

of land surface area. Therefore a DEM with a higher resolution should be used.

Another aspect resulting in lower significance can be found in the data of tectonic uplift. In this research, data of three different researches is used to create a tectonic uplift map for Crete. Thereby the assumption of constant tectonic movement over the past 18 thousand years is made. However, Crete has known multiple great earthquakes which resulted in vertical displacements in the Holocene (Strobl et al., 2014). To achieve more precise results, time specific tectonic uplift should be calculated and used in the equation.

The method used in this research to learn more about the possible habitat of prehistoric animals and early humans of Crete can lead to new knowledge on paleo biogeology of Crete. However, in this research the number of criteria used to describe the habitable area is only two. Only the slope and elevation of the area are considered. Since both humans and elephants have more limitations and preferences to their habitat, it would be interesting to use more elaborated criteria for future research. For example, both humans and elephants are tied to the proximity of fresh water.

4.4 Broader context and future research

Using the methods of this research could be interesting for research to visualise the effects of the sea level rise predicted by the IPCC. That way the future impacts of sea level rise on humans and other mammals can be predicted. IPCC has predicted global sea level rise with a rate of 3.2 mm/year between 2020 and 2050 (IPCC, 2013). The tectonic uplift shall probably be the largest other component influencing the relative sea level near Crete. If the tectonic uplift is to continue at the same rate, this would be between -0,5 and 4,8 mm/year. This would result in submerging of some areas of Crete. Furthermore this method can also be used to visualize how the sea level rise influenced other island or coastal areas since the last glacial maximum. To conduct research in another area or periods in time, new calculations to the isostatic and oceanic volume component should be made. This part is not described in this research but can be obtained from Koene (2013). Therefore it could be interesting to compose a model enabling a combination of both Koene’s method of calculating the relative sea level with the isostatic and oceanic volume component of a given location as well as the tectonic components. With such a model both the previous and future sea level changes and their impacts on human and animal habitats can be estimated.

4.5 Conclusion

Tectonic movements can have a great influence on the change of total land surface area. While relative sea level rise caused decrease in the total land surface area in the past 18 ka, the tectonic movements in the area of Crete resulted in increase of land surface area. The decrease in land surface area has especially affected the size of the habitable coastal areas. These coastal areas are also nowadays the areas with high population density. Gaining more knowledge on how the change in relative sea level can affect the area in which humans and other mammals live can be of great value, especially in light of the current climate change and the effects thereof.

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5. Acknowledgements

Kenneth van Rijsdijk, thank you very much for supervising me and giving me the time needed to finish this work. Thanks to Harry Seijmonsbergen for being my second examiner. For Thijs de Boer thanks for the technical assistance and feedback. Alexandra van der Geer, thank you for giving me inspiration and answering my questions. To all students and PhD’s in the GIS-studio thanks for answering my questions and keeping up the moral. Lissy-anne (Kipje) Denkers, Tom van der Meer, Adriaan Denkers, Anne Rietman, Siep Bakker, Ciska Bakker, Harry de Weijer and Justin de Jong thank you for feedback, answering my questions and keeping up my moral.

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7. Appendices

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Past present and future: Change of land surface area of Habitats on Crete since 18 ka