Climate change, lacustrine zone

INTRODUCTION

The climate history in the lacustrine zone of eastern Africa correlates to significant changes in earth's orbit and polar ice budgets. During the Holocene, regional lakes experienced a significant range of variability marked by abrupt transitions between high and low water level (Gasse 2000). Some of the variability can be attributed to mesoscale atmospheric changes to which it is subtly tuned due to its geography and position relative to the Indian and Atlantic oceans. The uneven topography of the region results in the condensation and precipitation of oceanic moisture at high elevations and rain-shadowing effect at lower elevations. Therefore, more or less inland flow (advection) of moisture has a profound impact on lacustrine water budgets as measured in lake sediments.

Paleoecological records mostly derive from laminated sediments deposited as erosional runoff (also called terrigenous), which trap microscopic remains of terrestrial organisms and their byproducts. These proxies are calibrated against modern, known data sets to provide insights into past temperature, rainfall, and vegetation conditions. If the sediments progressively build up over time (accrete), then they can be dated using radiocarbon or other radiometric techniques, which are statistically analyzed, or spline fitted, to model the passage of time between direct dates. Because of this, deep time records serve to provide baseline context for hydrologic changes observed in instrumental (historical) time, but the resolution of these records coarsens the further back in time one progresses (see Cohen 2018 for an overview of how such past-to-future models can be applied).

GEOGRAPHIC BACKGROUND

The East Africa Rift System is divided into eastern (a.k.a. Gregory) and western (a.k.a. Albertine) branches, extending from Lake Turkana to the north down to Lake Malawi in the south (see Figure 1). Rifting of the East African plate began perhaps as far back as 45 million years ago (Ma) when volcanic activity thinned the crust underlying the region, which was followed by warping and down-faulting of lithospheric blocks during the late Oligocene (Bergner et al. 2009). The formation of lakes in the northern part of the region dates to about 400,000 years ago, when significant rift-shoulder uplift processes accelerated on top of the East African Dome craton (Wolde-Gabriel et al., 2016). In the southern regions, basins such as Lake Malawi extend back into the Pliocene (4.5 Ma, Ring and Betzler 1995) and Lake Tanganyika to the Miocene (10 Ma, Wilson, Glaubrecht, and Meyer 2004).

Dip-slip and normal faults provide the geometry of the catchments' steep elevational gradients that orographically condense and precipitate moisture at high elevations draining into arid and semi-arid lowland basins. The lacustrine zone is therefore comprised of a series of endoheric and cascading lake systems with transient overflows depending on climatic conditions. With rare exception, lakes present today within this region have had water throughout the Holocene (beginning 9750 bce), although high and low stands have resulted in significantly different water balances throughout the epoch.

Tropical moisture pattern was dominated by the north-south movement of the Intertropical Convergence Zone (ITCZ). Northerly aspects of the region presently receive two rainy seasons, while the southernmost aspect receives a single, longer rainy season centered

Introduction

The climate history in the lacustrine zone of eastern Africa correlates to significant changes in earth's orbit and polar ice budgets. During the Holocene, regional lakes experienced a significant range of variability marked by abrupt transitions between high and low water level (Gasse, 2000). Some of the variability can be attributed to mesoscale atmospheric changes to which it is subtly tuned due to its geography and position relative to the Indian and Atlantic oceans. The uneven topography of the region results in the condensation and precipitation of oceanic moisture at high elevations and rainshadowing effect at lower elevations. Therefore, more-or less-inland flow (advection) of moisture has a profound impact on lacustrine water budgets as measured in lake sediments.

Paleoecological records mostly derive from laminated sediments deposited as erosional runoff (also called terrigenous), which trap microscopic remains of terrestrial organisms and their byproducts. These proxies are calibrated against modern, known data sets to provide insights into past temperature, rainfall and vegetation conditions. If the sediments progressively build up over time (accrete), then they can be dated using radiocarbon or other radiometric techniques, which are statistically analyzed, or spline fitted, to model the passage of time between direct dates. Because of this, deep time records serve to provide baseline context for hydrologic changes observed in instrumental (historical) time, but the resolution of these records coarsens the further back in time one progresses (see Cohen, 2018: for an overview of how such past to future models can be applied).

Geographic Background

The East Africa Rift System is divided into eastern (a.k.a. Gregory) and western (a.k.a. Albertine) branches, extending from Lake Turkana to the north down to Lake Malawi in the south (Figure 1). Rifting of the East African plate began perhaps as far back as 45 million years ago (Ma) which volcanic activity thinned the crust underlying the region, which was followed by warping and downfaulting of lithospheric blocks during the late Oligocene (Bergner et al., 2009). The formation of lakes in the northern part of the region dates to about 400,000 years ago, when significant rift-shoulder uplift processes accelerated on top of the East African Dome craton (WoldeGabriel et al., 2016). In the southern regions, basins such as Lake Malawi extend back into the Pliocene (4.5 Ma, Ring and Betzler, 1995) and Lake Tanganyika to the Miocene (10 Ma, Wilson et al., 2004). Dip-slip and normal faults provide the geometry of the catchments' steep elevational gradients that orographically condense and precipitate moisture at high elevations draining into arid and semi-arid lowland basins. The lacustrine zone is therefore comprised of a series of endoheric and cascading lake systems with transient overflows depending on climatic conditions. With rare exception, lakes present today within this region have had water throughout the Holocene (beginning 9750 BCE), although high and low stands have resulted in significantly different water balances throughout the epoch.

Tropical moisture pattern dominated by the north-south movement of the Intertropical Convergence Zone (ITCZ). Northerly aspects of the region presently receive two rainy seasons, while the southern most aspect receives a single, longer rainy season centered between November and March. The Congo Air Boundary (CAB) also traps warm Atlantic moisture westward. The shifting relative positions of these two pressure systems have greatly affected the distribution of effective moisture across the region throughout the Holocene (van der Lubbe et al., 2017). During periods when the eastern maximal position of the CAB shifts to the eastern margin of the Rift Valley, lakes are charged with water; when the eastern margin of the CAB is located west of the Rift, lake levels across the region are relatively low (Tierney et al., 2011). The dominance of atmospheric pressure systems in governing moisture makes the region particularly sensitive to changes in mesoscale atmospheric phenomena, such as the Indian Ocean Dipole and El Niño Southern Oscillation, which change the sea surface temperature dynamics responsible for most of the region's precipitation Jury et al., 2002).

The climate histories of the northern and southern lakes within the lacustrine zone are generally antiphased with each other throughout the Holocene (Castañeda et al., 2007). Thus, wet periods in the northern end of the Rift correlated with drier periods in the southern Rift and vice-versa (exceptions are noted below). This inverse climate phasing is attributable to relative positioning of the ITCZ, which displaces southward during glacial advance (progradation) periods in the extreme northern latitudes and northwards during glacial retreat (ablation) periods. Thus, the water budget moves in tandem with the ITCZ with lakes on the extreme margins of the ITCZ seeing particularly significant changes in level.

Climate History

High lake stand environments during the AHP had more biodiversity relative to low stand conditions, both within and adjacent to the lake margins. A detailed study of paleosols from the western shoreline of Lake Turkana indicates an extensive marsh-like riparian environment, which would have been attractive for endoaquatic foragers (Beck et al., 2019). A rapid transition to arid conditions is recorded after 3250 BCE in which lake level dropped 60+ m in a matter of a few decades (Figure 2; . On the other hand, Lake Victoria, which is the source of the White Nile River, experienced low water conditions from 7850 to 5550 BCE prior to an extended period of overfilling (Johnson et al., 2000). Figure 2. Lake proxies of largest significant water bodies of the East African Rift System: (a) Lake Turkana shoreline reconstruction , (b) Lake Victoria diatom record (Stager et al., 2017), (c) Lake Edward endogenic calcite record (Russell et al., 2003), (d) Lake Tanganyika C28 δ 13 C leaf wax record (Tierney et al., 2011), (e) Lake Malawi biogenic silica (diatom) mass accumulation rate (Johnson et al., 2002). The red line represents a 200-year rolling average. General cultural developments in regional prehistory are indicated at the top (HP = historic period, Ag./Iron = introduction and spread of plant cultivation and iron technologies, Past. = introduction and spread of pastoralism, Sedentary fisher-foragers = period in which high lake shorelines were inhabited by sedentary groups of fisher-foragers; see Marchant et al., 2018;.

At the southern distal aspect of the Rift Valley, Lake Malawi and smaller catchments adjacent to the lake, such as Lake Masoko, show the presence of an open landscape with low water levels during the zenith of the AHP (8000 -6000 BCE) (Barry et al., 2002;Garcin et al., 2007;Ivory et al., 2012).

Further north, at Lake Rukwa, pollen spectra are generally more in sync with records from the northern Rift showing closed woodlands associated with high rainfall conditions from the early Holocene through 3500 BCE after which grassy conditions become more prominent followed by significantly more arid, open conditions by 1500 BCE (Vincens et al., 2005). A similarly timed transition from closed woodland to open grassland is noted at Lake Tanganyika (Tierney et al., 2008) and Lake Kivu (Haberyan and Hecky, 1987), which defies the antiphased relationship inferred based on ITCZ positioning. Therefore, Tierney et al. (2008) posit that high sea surface temperatures in the western Indian Ocean present yet another mechanism for increasing regional precipitation in the southern Rift by increasing the strength of winter monsoon flow. Climate across tropical eastern Africa is highly variable and responds non-linearly to global-and mesoscale-forcing mechanisms.

Interconnected lacustrine systems are regionally abundant during the African Humid Period, which effectively ceased for eastern Africa by 2000 BCE (Tierney et al., 2011). There is a transition from deep, stratified water conditions in Lake Victoria to seasonally shallower waters until the advent of the Common Era, after which there is a more uniform water level within open, grassier conditions reflecting human impacts on the ecology of the catchment (Talbot and Laerdal, 2000). Human impacts on forest communities adjacent to Lake Kifuruka, located north of Lake Edward, can be discerned earlier, from 2350 BCE, suggesting the palynology of lakes in the region reflect complex interactions between anthropogenic and natural environments (Kiage et al., 2019). Pollen records from Lake Albert similarly suggest drier, grassier environments from 1650 BCE to present relative to early Holocene conditions (Beuning et al., 1997). Later Holocene records from Lake Masoko show wetter Zambezian woodland conditions until open, grassy conditions at 1450 and 800 BCE, between 300 and 500 CE and from 750 to 1450 CE, the latter two of which are attributed to anthropogenic disturbance of the forest (Vincens et al., 2003).

At Lake Edward, tropical woodlands dominated the shoreline until the abrupt termination of the AHP after which there was a gradual replacement by deciduous woodlands until the advent of the Common Era (Ivory and Russell, 2018). A ~725-yr climate cycle is recorded in the basin from 2950 BCE to 1550 CE, which correlates to variability in Indian Ocean monsoon strength (Russell et al., 2003). An arid period at the advent of the Common Era marks the arrival of the Iron Age, after which fire-tolerant vegetation and grasslands are indicative of anthropogenic disturbance more than climate (Ivory and Russell, 2018). There is an inferred positive feedback loop between the arrival of Iron Age agro-pastoralists in the Lake Victoria region associated with regional aridity, the spread of grasslands and burning (Battistel et al., 2016). However, anthropogenic burning in this region is argued to be more in tune with natural climate cycles (drier conditions have evidence of more burning) until the Colonial period, after which burning frequency is decoupled from climate conditions and increases significantly regardless of precipitation (Colombaroli et al., 2018).

High-resolution lacustrine records show significant lake level fluctuations on subdecadal scales throughout the Late Holocene so discerning regional climate signals from lake cores must be spatially and temporally contextualized based on the available data (Verschuren, 2004). Correlated regional droughts lasting 100 years or more have been identified from lakes Edward, Naivasha, Tanganyika and Turkana dating to 2150 BCE, 1 BCE/1 CE, 450 CE and 1100 CE (Russell and Johnson, 2005). Similarly, Lake Bogoria experienced shallow water conditions between 690 and 950 CE (De Cort et al., 2018). However, lake records from the "Little Ice Age" (ca. 1500 -1800 CE) suggest regionally heterogeneous responses to globally forced climate events. For example, sediment proxies from Lake Kitagata, adjacent to Lake Edward, show drought in the western Rift extension, while records in the eastern Rift experienced higher rainfall (Russell et al., 2007). Nevertheless, even within these records, a high-resolution, 1800-year lacustrine record from Lake Naivasha shows 20+m lake level rise during the latter half of the Little Ice Age (Verschuren, 2001) illustrating that the Little Ice Age is not a singular event across time and space. Historical records from travellers and oral histories fill in the gaps of paleoecological records, such as the late 19 th century droughts of the Bogoria/Baringo lake basin (Anderson, 2016). Such datasets demonstrate the need for close-interval sampling of lake cores, archaeological and historical data because coarse-resolution, spline fit ages models typically miss significant ecological variability.

Lake levels are sensitive to regional climate change and land use conditions. For example, a sediment proxy from Lake Baringo extending over the last 200 years shows ~50-year cycles of transgression and regression in tune with recorded regional droughts and wet periods (Okech et al., 2019). A similarly timed and synchronized relationship is noted in laminated deposits from Lake Tanganyika between 890 BCE and 530 CE (Cohen et al., 2006). However, the modern ecologies of both regions are overprinted by human activities, namely land clearance and deforestation, which are attributed to farming and pastoralism activities as well as urban development. Fishing is also an important aspect of the subsistence economy, and has been severely impacted by over-exploitation and climate change (Odada et al., 2003). The presence of invasive species in African Great Lakes has further degraded their biodiversity (Kasulo, 2000). Other human activities such as dam construction (Avery and Tebbs, 2018), ground water pumping for urban and agricultural pursuits (Gaye and Tindimugaya, 2019) and pollution/eutrophication (Twesigye, 2015) have also reduced water levels in recent decades (Cohen, 2018). Forecast projections from Great Lakes predict significant changes in hydrology, which could shift resources and negatively impact the economic livelihoods of the region's inhabitants (Beverly et al., 2020).

Conclusion

Understanding the long-term ecological context of the lacustrine zone of eastern Africa informs better understandings of archaeological activities as well as provides insights into future scales of landscape change. Within the framework of increased emissions of greenhouse gasses and reductions in biodiversity due to wholesale landscape clearance, the evolutionary relationship between climate and landscape is of critical importance to be able to attenuate negative effects of ecological change. Similarly, past human activities were not exclusively positive or negative in how they impacted biodiversity. Humans are organisms that respond to climate change by shaping their local environments. When combined with the actions of others across wide geographic regions, these impacts can blur the lines between natural and anthropogenic ecological and climatic change.