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Holocene lacustrine fluctuations and deep CO2 degassing in the northeastern Lake Langano Basin (Main Ethiopian Rift)
Journal of African Earth Sciences 77 (2013) 1–10
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Holocene lacustrine fluctuations and deep CO2 degassing in the northeastern Lake Langano Basin (Main Ethiopian Rift) Marco Benvenuti a,b,⇑, Marco Bonini b, Franco Tassi a, Giacomo Corti b, Federico Sani a,b, Andrea Agostini a, Piero Manetti a,b, Orlando Vaselli a a b
Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira 4, 50121 Firenze, Italy C.N.R, Istituto di Geoscienze e Georisorse, UOS Firenze, Via G. La Pira 4, 50121 Firenze, Italy
a r t i c l e
i n f o
Article history: Received 24 October 2011 Received in revised form 14 August 2012 Accepted 2 September 2012 Available online 20 September 2012 Keywords: Main Ethiopian Rift Holocene Lacustrine fluctuations Carbon geochemistry Active tectonics
a b s t r a c t This work reports the results of an integrated investigation on Holocene faulted deposits exposed on the northeastern Lake Langano in the central sector of the Main Ethiopian Rift (MER). The Lake Langano is part of a closed basin and thus it is highly sensitive to climate fluctuations. The present study explored the foot of the Haroresa escarpment, where coarse-grained slope deposits interbedded with thin lake shore shell-rich coarse sands are well exposed. Stratigraphical analysis of these deposits, integrated with radiocarbon dating carried out on lacustrine gastropod Melanoides tubercolata shells, points to significant lake level fluctuations forced mainly by climate oscillations. These have been interplaying during the early and middle Holocene with a structurally-controlled threshold separating an embayment of the Lake Langano to the south, from the small perched Lake Haro Bu-a basin to the north. Significant differences in the 13C/12C isotopic ratio have been identified in the M. tubercolata shells collected on the opposite sides of such a threshold. The time-space variations of the isotopic signature of the shells are referred to the mutual relationships between the two main different CO2 sources (i.e., microbial activity and deep mantle degassing) dissolved in the lake water. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
2. Hydrologic evolution of the Lakes Region
Researches on the Late Pleistocene–Holocene evolution of the Lakes Region in the Main Ethiopian Rift (Ethiopia), have concurred to update regional models of hydrologic responses to global climatic changes, providing a reference for the late Quaternary palaeoclimatology of the tropics (Street-Perrott and Perrott, 1990; Gasse and Van Campo, 1994). Along with such a palaeoclimatic relevance, the MER is also a zone characterized by active tectonic processes and intense hydrothermal activity (Gianelli and Teklemariam, 1993; Chernet, 2011). In such a dynamic setting, climatic, tectonic and magmatic-related processes may have exerted a concurrent and complex control on the Late Quaternary geomorphic and depositional dynamics. We focus on the northern coast of Lake Langano where lacustrine shells-bearing deposits and their radiogenic and stable carbon isotopic signature record Holocene, climate-driven, lake level fluctuations in the frame of a local active tectonic setting.
The Lakes Region includes lakes Ziway, Langano, Abiyata and Shala that developed since the Late Pleistocene (Street, 1979; Benvenuti et al., 2002) within the fault-controlled Main Ethiopian Rift depression (Fig. 1). Classically, the Holocene Ziway–Shala lake fluctuations (Street, 1979; Gasse and Street, 1978; Gillespie et al., 1983; Alessio et al., 1996; Fig. 2) have been considered a suitable proxy for the switching off and on of the monsoonal regime, as a regional response to global cooling and warming trends (StreetPerrott and Perrott, 1990; Gasse and Van Campo, 1994). Between 10 and 5 ky (conventional 14C ages), the lakes experienced an overall high-level status abruptly interrupted by short arid pulses. The early-middle Holocene (5–10 ky) wetter period coincides in the subarid tropics with a climatic optimum attesting to a relatively stable monsoonal circulation over East Africa. Short-lived arid pulses are related to a weakened monsoonal circulation which, in some cases, correlated quite well with global cooling events (Benvenuti et al., 2002). Since 5 ky to Present, the lakes progressively reduced in size due to increasing aridity. The progressive hydrological deficit over the Lakes Region during the late Holocene (post5 ky) may be related to increasing warming over the last 1000 years. Despite the evolution of the Ziway–Shala lacustrine system has been generally referred to a dominant climatic control,
⇑ Corresponding author at: Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira 4, 50121 Firenze, Italy. Tel.: +39 0552757516; fax: +39 055218628. E-mail address: [email protected]fi.it (M. Benvenuti). 1464-343X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jafrearsci.2012.09.001
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Fig. 1. Regional setting of the Lakes Region of the Main Ethiopian Rift. (a) Schematic tectonics setting of the East African Rift System; MER: Main Ethiopian Rift. (b) Detail of the Lakes Region in the central MER, showing the main Late Pleistocene–Holocene depositional units and their volcanic bedrock (after Benvenuti et al. (2002)). HRFS: Haroresa Rhomboid Fault System. Stars and related numbers refer to the location of lacustrine shell samples in Table 1.
120
height of overflow channel ?
?
Pleistocene, despite the early-middle Holocene climate was wetter than the last glacial (Benvenuti et al., 2002; Sagri et al., 2008).
?
Meters above the SD
3. Tectonic setting
SD (1558 m asl)
80
?
40
0 14
? 10
6
Age (C
14
2
0
kyr BP)
Fig. 2. Lake level fluctuations in the Lakes Regions (after Gillespie et al. (1983)). SD: Lake Shala datum 1558 m. a.s.l (1970s gauge data).
the tectonic setting of an active continental rift has played a significant role in controlling the surface hydrology as suggested for other lakes within the East African Rift System (e.g., Bergner et al., 2009). Around the Pleistocene–Holocene boundary, tectonically-driven stream piracy cut down a significant water budget to the lakes that was supplied by the northern catchments of paleo-Awash and paleo-Mojo rivers (Sagri et al., 2008). This determined a lake system less extended in the Holocene than in the Late
The Main Ethiopian Rift (MER) is part of the largest East African Rift System, and extends between the Afar depression to the North and the Kenya Rift to the South (Fig. 1a). The MER is characterized by a roughly NE-trending rift valley striking close to N–S in its southern sector which has been developing discontinuously since the Late Miocene in response to the extensional stress produced by Nubia–Somalia movement (e.g., Corti, 2009). The resulting fault pattern consists of two main fault systems: (1) a border fault set, and (2) a set of internal faults commonly referred to as Wonji Fault Belt (WFB; Mohr, 1962) that are best expressed in the northern MER. In the central MER, the border faults formed at 6–8 Ma (WoldeGabriel et al., 1990; Bonini et al., 2005), while the WFB faults started developing since about 2 Ma (Boccaletti et al., 1998; Ebinger and Casey, 2001), and are still in an incipient stage (Agostini et al., 2011). By controlling the rift floor morphology, the WFB activity gave rise to an interplay between climatic-forced lake level fluctuations and tectonics (e.g., Benvenuti et al., 2002). The northeastern Lake Langano, which is the focus of this paper, is adjacent to the eastern rift margin, which is typically characterized by a peculiar rhomboidal arrangement of faults with characteristic zigzag geometry, referred to as Haroresa Rhomboid Fault System (e.g., Le Turdu et al., 1999). This system essentially affects the Pliocene rift floor ignimbrites and locally the Middle Pleistocene–Holocene colluvial–alluvial deposits and pyroclastics (Dainelli et al., 2001). Such a recent fault activity probably
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controlled the emplacement and circulation of hydrothermal fluids associated with the Aluto-Langano geothermal field (Gianelli and Teklemariam, 1993), where volcano-tectonic activity is also documented by satellite imagery analysis (Biggs et al., 2011). The tectonic evolution of the MER was preceded by intense volcanic activity, which started with the eruption of Oligocene (30– 35 Ma) voluminous flood basalts (e.g., Mohr and Zanettin, 1988; Ebinger et al., 1993; WoldeGabriel et al., 1990; Chernet et al., 1998; Fig. 1). Subsequent magmatism was marked by a second episode of pre-rift flood basalts at 8–12 Ma (Abebe et al., 2005; Bonini et al., 2005), followed by the eruption of widespread rhyolitic ignimbrites (Rift Floor ignimbrites or Nazret Pyroclastic Rocks of Abebe et al., 2005; Fig. 3) that marked the onset of rift activity in the Late Miocene. After this widespread Mio-Pliocene volcanism, the Quaternary magmatic activity mostly localized in the rift floor, with products showing a typical bimodal composition given by alternating basaltic (e.g., Wonji basaltic activity; Wonji Basalts of Abebe et al., 2005; Fig. 3) and rhyolitic (e.g., Aluto-Bericcio products; Bora-Bericha Rhyolites of Abebe et al., 2005; Fig. 3) activity mostly associated with the WFB faulting.
4. Stratigraphic and paleogeographic setting of the Late Quaternary continental deposits Following to the seminal studies of Street (1979), Gasse and Street (1978) and Gillespie et al. (1983), in the last decade a stratigraphic revision of the Late Quaternary succession, covering Pliocene–early Quaternary volcanic and volcaniclastic deposits (Abebe et al., 2005), allowed to trace four unconformity-bounded units (ISSC, 1994; synthems 1–4) over the Lakes Region that record major stages in the lake evolution (Fig. 1b; Benvenuti et al., 2002). In the northern Lake Langano area, the Late Pleistocene–Holocene
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lacustrine succession overlies reddish alluvial–colluvial conglomerates, sandstones and mudstones ascribed to the latest Middle Pleistocene (Coltorti et al., 2002). The synthems composing the Late Quaternary deposits are briefly described from bottom to top (see Fig. 1). Synthem 1 consists of colluvial, fluvio-deltaic, lacustrine gravels, sands, and muds, diatomites and volcaniclastic materials, deposited during the last glacial period, ca. 100,000 to 22,000 yrs BP (Megalake phase; Benvenuti et al., 2002; Fig. 1). This latter lake system expanded during wetter interstadials and contracted during the drier stadials. Synthem 2 is dominated by alluvial–colluvial and volcaniclastic deposits accumulated during the last full glacial and late glacial, ca. 22,000–10,000 yrs BP when arid conditions brought to severe shrinking of the lakes (Reduced Lake Phase, Benvenuti et al., 2002; Fig. 1). Synthem 3 consists of colluvial, fluvio-deltaic, lacustrine gravels, sands, and muds, diatomites, and volcaniclastic materials recording the early-middle Holocene, ca. 10,000– 5000 yrs BP (Macrolake phase of Benvenuti et al. (2002); Fig. 1). This lake system, smaller than the former Megalake, underwent frequent level fluctuations (Benvenuti et al., 2002; Benvenuti et al., 2005) with a maximum lake surface elevation of about 1670 m above sea level (Street, 1979; Laury and Albritton, 1975; Benvenuti et al., 2002). Synthem 4 consists of colluvial, fluvial, deltaic and lacustrine sediments accumulated over the last 5000 years when Macrolake progressively split into the present four lakes (Separated Lakes Phase of Benvenuti et al. (2002); Fig. 1).
5. The Haroresa road cross-section In February 2007, a shallow ditch excavated along the steep slope delimiting the eastern escarpment of a faulted block on the northeastern coast of the Lake Langano (Haroresa escarpment;
Fig. 3. Geological map of the northern Lake Langano basin (adapted from Dainelli et al. (2001)).
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Figs. 3, 4a and 5) allowed the direct observation and sampling of a few meter thick clastic succession recording a mix of slope and lacustrine depositional settings (Fig. 3). The excavation also brought to light a fault zone through-going these deposits, which was also surveyed in February 2009 (Fig. 5a and b). 5.1. Structural setting The northeastern corner of the Lake Langano lies within the Haroresa Rhomboid Fault System (HRFS), which has resulted from the intersection between two fault trends: a dominant NNE to NEtrending fault set, and a minor N-to-NNW-trending fault set. These fault systems gave rise to horst, graben and half-graben structures mainly trending around NNE to NE, with the principal river courses flowing within the elongated depressions. The northeastern Lake Langano corner is influenced by both HRFS fault sets (Fig. 3). In the Haroresa-Haro Bu-a area, the dominant NNE- to NE-trending and southeast-dipping normal faults produce several major northwest-dipping tilted blocks forming a series of 0.5- to 3-km-wide half-grabens (Figs. 3 and 4b; Le Turdu et al., 1999). These depressions are filled by Quaternary sediments or are occupied by small lakes or swamps onlapping onto the major bounding fault scarps, such as for the Haroresa and Haro Bu-a escarpments (Figs. 3 and 4). The fault scarps are 50–150 m-high, generally steep (>60°), and exhibit a fresh morphology (Fig. 4). The Lake Haro Bu-a is located in the overlapping zone between two NNE-trending faults that join southwestward into a single fault, which is magnificently exposed by the Haroresa road crosssection. The fault separates the Pliocene rift floor ignimbrite on the fault footwall from the clastic succession made of coarse grained slope debris and lacustrine deposits bearing abundant Melanoides tubercolata shells (see below). The fault zone is characterized by closely spaced shear planes affecting the Holocene clastic succession (Fig. 5a and b), thereby demonstrating ongoing fault
activity that is also coherent with its prominent morpho-tectonic signature (Fig. 4). The fault planes record evident striations that allowed the measurements of a number of good quality fault-slip data. The striations show a main pure to slight oblique-slip kinematics that is consistent with a local ca. E–W-trending Holocene extension direction (Fig. 5a). In agreement with these results, fault-slip data collected at sites distributed on this sector of the eastern rift margin indicate an overall N100°E-directed Quaternary extension (Agostini et al., 2011). These data accord well with previous paleostress determinations carried out on the same rift margin (Bonini et al., 2005; Pizzi et al., 2006) as well as with the results of geodetic analyses (Bilham et al., 1999; Stamps et al., 2008). 5.2. Geomorphology and stratigraphy The outcrop is exposed for some hundred meters over the morpho-tectonic landscape related to the HRFS (Figs. 3 and 4). The studied sections are located just at the transition between the isolated Lake Haro-Bu-a and the Langano embayment (Fig. 4). The Haroresa escarpment consists of three distinct sectors. The midupper slope is made of plane-bedded Rift Floor Ignimbrites, which determine a stair-step morphology of the slope. In the mid-lower portion two differently inclined surfaces occur. The steeper and higher one subtends boulder-sized colluvial deposits delimited on top by a flat surface representing a lacustrine terrace (Fig. 4a). On the basis of available elevation data, the current altitude of this terrace stands between 1660 and 1680 m above sea level, which falls in the range of the maximum early-middle Holocene Macrolake level. The lower slope surface, cut by the road excavations, is less inclined and exposes a mix of coarse and fine-grained debris, which represents the target of more detailed observations. Stratigraphic description and sediment sampling for radiocarbon analysis were carried out on two distinct outcrops (sites 1 and 2)
Fig. 4. (a) View of the Haroresa fault escarpment. The white boxes indicate the stratigraphic sections 1 and 2 described in the text. (b) Lateral view of the northern segment of the Haroresa fault escarpment downthrowning Lake Haro Bu-a and tilting the lacustrine deposits. The open black arrow indicates the Haroresa normal fault. The open white arrows indicate the fault-bounded tilted blocks. The tip of white triangles indicate the base of the morphologic fault scarp. Photo view points are indicated in Fig. 3.
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Fig. 5. Structural and stratigraphic features along the Haroresa escarpment. (a) Major fault plane (indicated by white arrows); the stereonet on the top-right handside shows the collected fault slip data (Wulff net lower hemisphere; after Agostini et al. (2011). (b) Detail of the fault zone with a series of parallel shear planes (indicated by white arrows) affecting the slope debris and Holocene lacustrine sediments. (c) Slope deposits with minor lake shore shell-rich pebbly sands exposed on the fault hanging-wall (persons for scale); 1a and 1b indicate the samples collected for radiocarbon dating (see Table 1). (d) Close-up of the shore pebbly sand-bearing shells (gastropod M. tubercolata) collected at site 1a.
separated by a saddle separating the Langano embayment from the Lake Haro-Bu-a (see location in Fig. 4a).
sediments during lake transgressions and gravity transport during regressive stages.
5.2.1. Site 1 The clastic succession exposed at site 1 on the hangingwall of the Haroresa normal fault (visible in Fig. 5a), consists of about 5– 6 m thick blocky angular clasts deriving from the Rift Floor Ignimbrites, and subordinate pebbly sands (Fig. 5b and c). In the lower part of the outcrop, pebbly sands rich in freshwater mollusk shells, dominated by the gastropod M. tubercolata (Fig. 5c and d), attest to a lacustrine nearshore environment. These deposits are overlain by crudely layered cobble-boulder sized gravel dispersed in a variable amount of finer matrix. Clast composition is monogenic and made of the Rift Floor Ignimbrites. Textures of these deposits ranges from clast- to matrix-supported (Fig. 5c). On the whole, these deposits are referred to debris fall and debris flow that occurred along the steep Haroresa escarpment. In specific clast-supported beds, the boulders are coated by a few millimeter-thick withish massive to laminated carbonate crust (Fig. 5c). Two samples were collected for radiocarbon analysis (Fig. 5c). Sample 1a includes several shell specimens of M. tubercolata, sample 1b is a single clast whose carbonate coating was separated in laboratory (Table 1). The depositional setting at site 1 oscillated between a lacustrine rocky coast confining the Langano embayment during lake high-stand and a subaerial slope during lake regression. Therefore, the operating depositional processes were the biogenic carbonate precipitation and wave reworking of the clastic
5.2.2. Site 2 The ditch excavated on the upslope side of the road at site 2, a few tens of meters from site 1 (Fig. 4a), shows a quite different succession with respect to that of site 1. Despite its limited thickness, this succession is characterized by a more articulated architecture (Fig. 6a). In particular, the basal deposit is a massive pebbly–cobbly gravels with abundant sandy–silty matrix (Fig. 6b). The basal deposit is overlain by pebbly sands with abundant mollusk shells dominated by M. tubercolata indicating a paleoshore at the southern end of the Lake Haro Bu-a. A thin fine-grained ash bed rests over the pebbly sand deposit, and it is unconformably overlain (Fig. 6a and b), by a deposit consisting of pebbly sands, reworked ash and abundant M. tubercolata shells. Both Melanoides-bearing beds onlap basal massive gravels, and outline the southward termination of the Haro Bu-a lacustrine area (Fig. 6c). Such a marginal lake succession is overlain unconformable by a thick matrix-rich gravels sustaining a modern dark stony soil (Fig. 6a and c). Two samples (2a and 2b) of several specimens of Melanoides shells have been collected for radiocarbon dating, respectively below and above the angular unconformity separating the two layers of Melanoides-bearing sands (Fig. 6b; Table 1). Depositional processes were dominated by gravel transport in short creeks draining the slopes during the regressive stages, and by wave reworking in a nearshore environment during high-lake
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Table 1 13 12 C/ C isotopic ratios and conventional ages of samples of Melanoides and/or mixed lacustrine shells collected from different portions of the Ziway–Shala basin (location in Fig. 1b). Radiocarbon dating (materials measured by the standard radiometric technique; see http://www.radiocarbon.com for details) has been carried out in this study (samples 1a, 1b, 2a, 2b) and during 1995–1998 (samples 311) by both the Beta Analytic Radiocarbon Dating Laboratory (Miami, Florida, USA) and CNR-Istituto per le Tecnologie Applicate ai Beni Culturali (Rome, Italy). The radiocarbon ages of samples 3–11 are published in Benvenuti et al. (2002), whereas the correspondent 13C/12C isotopic ratios are unpublished. Sample
Laboratory code
13
C/12C
Conventional age
Latitude
Longitude
1a 1b 2a 2b 3 4 5 6 7 8 9 10 11
Beta 231103 Beta 263657 Beta 231101 Beta 231102 Beta 114024 R-2425 R-2426 R-2441 R-2444 Beta 114023 Beta 098943 Beta 114021 Beta 114022
3.3 o/oo +3.8 o/oo 5.0 o/oo +0.2 o/oo 1.7 o/oo 3.3 o/oo 2.5 o/oo 2.8 o/oo 3.0 o/oo 4.3 o/oo 3.1 o/oo 2.8 o/oo 2.3 o/oo
5840 ± 60 BP 7190 ± 60 BP 9730 ± 60 BP 6350 ± 50 BP 5820 ± 60 BP 5992 ± 43 BP 7642 ± 43 BP 10,347 ± 79 BP 11,590 ± 203 BP 27,040 ± 260 BP 30,440 ± 160 BP 39,170 ± 760 BP 47,470 ± 1600 BP
7°400 22.9400 N 7°400 22.9400 N 7°400 26.6900 N 7°400 26.6900 N 7°280 10.4400 N 7°470 28.3400 N 7°480 44.0600 N 7°480 44.0600 N 7°480 44.0600 N 7°470 28.3400 N 7°570 45.2300 N 7°570 45.2300 N 7°470 28.3400 N
38°480 37.8700 E 38°480 37.8700 E 38°480 36.9400 E 38°480 36.9400 E 38°380 51.4700 E 38°410 46.4700 E 38°410 39.4400 E 38°410 39.4400 E 38°410 39.4400 E 38°410 46.4700 E 38°390 50.6700 E 38°390 50.6700 E 38°410 46.4700 E
levels. The thin ash beds record syndepositional pyroclastic fall in the Haro Bu-a basin attesting to volcanic activity in the surrounding. 5.3. Carbon radiogenic and stable isotope geochemistry Samples 1a–b and 2a–b were analyzed for the determination of the radiocarbon ages at the Beta Analytic Laboratories according to AMS and standard techniques (see http://www.radiocarbon.com/ for analytical details). These shell samples, together with lacustrine shells collected from nine sites located in different portions of the broader Ziway–Shala basin – i.e., along the shoreline of Lake Shala (sample 3), west of Alutu Mt. (samples 4, 5, 6, 7, 8, and 11) and west of Lake Ziway (samples 9 and 10) (Fig. 1b) were analyzed for the determination of the 13C/12C ratios following the procedure proposed by McCrea (1950). The 14C data (Table 1) indicate that the studied sections are fully Holocene in age, allowing the correlation of the local stratigraphy to synthems 3 and 4 established in the whole Ziway–Shala basin. Ages of samples 1a and 1b outline a marked stratigraphic inconsistency, which could be referred to the following evolution: (i) algal carbonate coating of upslope colluvial deposits during the maximum Megalake highstand at about 1670 m a.s.l. (marked by the lacustrine terrace in Fig. 4a), and (ii) downslope re-deposition of the coated clasts (i.e., sample 1b) during regressive phases postdating the deposition of shoreline deposits (i.e., sample 1a). Samples 2a–b yielded radiocarbon ages consistent with their mutual stratigraphic position. In spite of a very close 14C age, samples 1a and 2b have different 13C/12C ratios (3.3 and +0.2 permil V-PDB, respectively; Table 1). The 13C/12C ratios of samples 3–11, whose radiocarbon age range from 47,000 to 5000 years BP (Benvenuti et al., 2002), define rather similar negative values ranging from 1.7 to 4.3 permil V-PDB (Table 1). 6. Discussion The stratigraphic analysis confirmed the role of environmental changes as mainly driven by climate variations. Late Quaternary tectonics and volcanism in the MER modulated such a climatic control by forcing geometric changes in the basin and determining a significant variation in its areal extension, as manifested by the transition from the late Pleistocene Megalake system to the early-mid Holocene Macrolake system (Fig. 1). Even assuming slip-rates up to 1 mm/yr for the active faults, the resulting uplift rates are not comparable with the meters-scale climatic-forced lake level fluctuations occurring at least one order of magnitude faster. Nevertheless, the studied sections provide interesting elements for integrating sedimentary, tectonic and geochemical lines
of evidence of the Holocene evolution of both the Langano embayment and the adjacent Haro Bu-a basin.
6.1. Stratigraphic and tectonic constraints on the palaeohydrologic dynamics of the Langano embayment-Haro Bu-a basin The road-cut from site 1 to site 2 crosses the modern fault-controlled threshold separating the Lake Langano from the Haro Bu-a basin, which also acted as a significant morpho-structural, hydrological and geochemical boundary during the great part of the Holocene. Such a threshold is the expression of an active tectonic setting of the HRFS (Le Turdu et al., 1999). The fault plane brought to light by excavation at site 1, documents a late Holocene (post 6200 14C years BP; sample 1a) activity of the fault system giving rise to the Haroresa escarpment (Agostini et al., 2011). From a stratigraphic point of view, the sections exposed at sites 1 and 2 reflect at a local scale the general evolution of the Holocene Ziway–Shala lakes basin (Benvenuti et al., 2002): early-mid Holocene synthem 3 and late Holocene synthem 4 left clear traces as marginal lacustrine and slope deposits, respectively (Figs. 5 and 6). As regards the synthem 3 deposits, the comparison between the elevation/chronology of the shell beds 1a, 2a, and 2b and the overall Holocene lake fluctuations (Gillespie et al., 1983), locate the dated sediment during stages of rising or high-lake levels (Fig. 2). The reconstructed lake level fluctuations across the saddle are schematically illustrated in Fig. 7. The available topographic data allows to place the present threshold separating the Langano embayment from the Haro Bu-a lake around 1650 m. a.s.l., with the shell samples (1a, 2a and 2b) being collected a few meters below the threshold (Figs. 4a and 7). Both the sedimentary features observed in the sampled shell beds and the stratigraphic onlap at the site 2 section testify to the deposition in nearshore environments. These latter pertained to the Lake Langano (sample 1a) and Lake Haro Bu-a (samples 2a–2b), which were thus separated at about 9730 14C years BP (sample 2a; Fig. 7a) and around 6200–6300 14C years BP (samples 1a–2b; Fig. 7c; Table 1). A higher lake level is indirectly recorded by the 7190 14C years-old thin stromatolite coating some boulders in the slope deposits of synthem 4 at site 1 (sample 1b; Fig. 7b). The stromatolite evidences a high lacustrine stillstand that is morphologically recorded by a terrace cutting into the Haroresa escarpment at around 1670 m a.s.l,, and that likely corresponds to the maximum level of the early-mid Holocene Macrolake. In agreement with Gillespie et al. (1983), the Ziway–Shala lakes level was rather high at around 7000 14C years BP, and was possibly approaching its maximum level that reached between 6000 and 5000 14C years BP (Fig. 2). Under this hypothesis, the Langano embayment and the Lake Haro Bu-a may
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Fig. 6. (a) Stratigraphy of the site 2 section: early Holocene shore and slope deposits, light in color, are overlain by recent brownish matrix-rich boulders accumulated at the toe slope of the Haroresa escarpment facing the lake Haro Bu-a basin (see Fig 4). Black arrows outline the southward stratigraphic onlap of the upper Haro Bu-a shore deposits onto an ash bed topping the lower shore deposits and the underlying slope deposits. The rod is 1 meter long. (b) Detail of the area enclosed by the white box in (a) showing the stratigraphy described in the text together with the location of samples 2a and 2b, which consist of several specimens of M. tubercolata collected for radiocarbon dating (see Table 1). The thick black dashed lines indicate unconformities. (c) Pinch-out of the lake shore deposits toward the south, i.e., approaching the threshold separating the Langano embayment from the isolated lake Haro Bu-a (see Fig. 4a); the thick black dashed lines indicate unconformities.
have been joined about 7190 14C years ago (Fig. 7b). In hydrological terms, the story reconstructed from sites 1 and 2 stratigraphy tells about a dominant isolation of the Lake Haro Bu-a from the Langano embayment, interrupted by a short-lived connection during lacustrine highstand around the mid Holocene maximum lake level (Fig. 7). A NE-dipping angular unconformity separates the lower and the upper mollusk-bearing paleo-shore deposits (Fig. 6), which correspond to the two major Lake Haro Bu-a highstands at ca. 9730 14C years BP (sample 2a) and around 6200–6300 14C years BP (sample 2b; Table 1). Such a bottom lake tilting is consistent
with an event of tectonic subsidence affecting the fault-controlled Haro Bu-a basin. Both faulting evidence and the presence of ash fall deposits across the unconformity attest to an episode of volcanotectonic activity during early-middle Holocene. 6.2. Endogenic vs. exogenic isotopic signatures of Carbon in Melanoides shells The carbon geochemistry, derived from the dating of carbonate organic remains collected at sites 1 and 2, reveals an intriguing variation in the 13C/12C isotopic ratio of the M. tubercolata shells.
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Fig. 7. Schematic cartoon showing the reconstructed 3-step lake-level fluctuations of the Langano-Haro Bu-a threshold in relation to the supposed post9730 yr BP increase in mantle-derived CO2 supply. The position of dated samples is shown, together with the Conventional Radiocarbon Age and the 13C/12C ratio (in parenthesis). See text for details.
Owing to the different basin dimensions, distinct environmentalgeochemical conditions (e.g. water temperature and pH) could have been established within the Lakes Haro Bu-a and Langano. In particular, these conditions might have caused different carbon isotopic signatures of CO2, and, consequently, of bicarbonate involved in the cycle of the biotic calcification process of M. tubercolata, as described by the following reaction (Hotchkiss and Hall, 2010):
Ca2þ þ 2HCO3 ! CaCO3 þ H2 O þ CO2 Therefore, a variation of the 13C/12C ratios of CO2 dissolved in the Lake Haro Bu-a water may have indirectly caused a change of the 13 12 C/ C ratios of the M. tubercolata shells, which used bicarbonate ions to construct their exoskeletons. By reference to the lake water, the CO2 sourced from within a system, such as that produced by microbial activity, can be referred to as ‘endogenic’; the CO2 coming from outside the lake system, such as the mantle, is instead ‘exogenic’. It is worth mentioning that the 13C/12C value of mantle-related CO2 is typically heavier (from 7 to 3‰ V-PDB) than that of microbial CO2 (<20‰ VPDB) (Rollinson, 1993; Hoefs, 1997; Ohmoto and Goldhaber, 1997). Deep-originated CO2 has been likely discharged into the Lake Langano system through fluid emissions related to the strong volcano-tectonic and geothermal activity characterizing this sector of the MER (Gianelli and Teklemariam, 1993; Biggs et al., 2011).
The establishment of isolation phases of the Lake Haro Bu-a from the Lake Langano are revealed by the paleo-shore deposits at site 2 (Figs. 6 and 7). The shells collected in the two distinct paleo-shore layers show marked difference in the 13C/12C ratios (samples 2a and 2b; Fig. 6 and Table 1). This isotopic difference could be explained by variations in (i) environmental conditions (influencing biogenic CO2 production occurring within the lake) and/or (ii) hydrothermal activity (regulating the exogenic CO2 contribution). The shells contained in the older paleo-shore beds (sample 2a), deposited during an early phase of isolation, yield a negative 13 12 C/ C ratio (5‰ V-PDB) that correlates regionally with the negative values of the 13C/12C isotopic ratio determined on shells of samples 3–11 (Table 1) representative of the Mega- and Macrolake systems (Benvenuti et al., 2002, and unpublished data; Fig. 1). In this scenario, sample 2b is the only showing a positive isotopic ratio (13C/12C = +0.2), implying that the isolated Lake Haro Bu-a recorded a major change in the isotopic composition of the lake water between 9730 yr BP (sample 2a) and 6350 years BP (sample 2b). Such a positive 13C/12C ratio could manifest the input of relatively high amounts of isotopically heavy mantle-related CO2 into the system, which has strongly affected the carbon isotopic signature of carbon-bearing dissolved ions (mainly HCO3) of the small Lake Haro Bu-a. The larger Lake Langano would have in fact buffered any exogenic deep-input of 13C-rich CO2 because microbial activity represented the main (endogenic) CO2 source controlling the negative isotope ratio of the carbon-bearing ions of the lake and, consequently, of the shell composition (cf. the 13C/12C ra-
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tios of the roughly coeval samples 1a and 2b; Table 1). In other terms, the isolation of the Lake Haro Bu-a, in combination with the recorded variation in the 13C/12C ratio in the M. tubercolata shells collected within the Lake Haro Bu-a basin (samples 2a and 2b), have allowed the recognition of a significant variation in the tectono-magmatic activity in this rift portion during early-middle Holocene times. The effects of evaporative processes operating in the different sized Lakes Langano and Haro Bu-a cannot be invoked to explain the difference between the isotopic values of the samples 1a and 2b, because a very different carbon isotopic signature of shells has been recorded during the two distinct isolation events of the Lake Haro Bu-a (i.e., samples 2a and 2b; cf. Fig. 7a with 7c). As for as the 13C/12C ratio of stromatolites (sample 1b) is concerned, this cannot be directly compared with those of the M. tubercolata shells, because the carbon isotope fractionation related to the biocalcification processes of these two types of living organisms may be different. From the above discussion, we interpret the presented data as evidence that the input of deep exogenic CO2 into the geothermal system of the Lake Langano increased between 9730 years BP (sample 2a; Fig. 7a) and 6350 years BP (sample 2b; Fig. 7c). The network of rift faults described in the area has been shown to consist of several active structures, which thus represent the most likely candidate that accomplished such a supposed post9730 year BP transfer increase of mantle fluids to shallower levels. 7. Concluding remarks This integrated work has allowed to put forward the following main points: (1) Geochemical and radiocarbon dating of lacustrine shells, corroborated by detailed stratigraphic field analysis, have allowed reconstructing local variations in Lake Langano level during early-middle Holocene (5–10 ky), which accord with, and may refine, the reconstructed regional hydrologic fluctuations. (2) Local active faults may bear a two-fold importance by providing both (1) the topographic frame onto which the lake level fluctuations have been operating, and (2) potential deeply-rooted fault-controlled pathways exploited by ascending fluids. (3) Finally, the present work may offer a case history in which lake-level fluctuations modulated by climatic forcing combine with variations in the supply of CO2-rich fluids sourced from the mantle up-through active faults.
Acknowledgments Research supported by CNR Funds (RSTL No. 105 ‘‘Evoluzione della parte Nord del rift Afroarabico e distribuzione regionale delle georisorse’’; P.I.M. Bonini). References Abebe, T., Manetti, P., Bonini, M., Corti, G., Innocenti, F., Mazzarini, F., 2005. Geological map (scale 1:200,000) of the northern Main Ethiopian Rift and its implication for the volcano-tectonic evolution of the rift. Geological Society of America Map and Chart Series, MCH094. Agostini, A., Bonini, M., Corti, G., Sani, F., Manetti, P., 2011. Distribution of quaternary deformation in the central Main Ethiopian Rift. East Africa, Tectonics, 30, TC4010. http://dx.doi.org/10.1029/2010TC002833. Alessio, M., Allegri, L., Belluomini, G., Benvenuti, M., Cerasoli, M., Improta, S., Manfra, L., Sagri, M., Ventra, D., 1996. Le oscillazioni tardo-quaternarie del Lago Shala (Rift Etiopico): Analisi dell’evoluzione ambientale dall’integrazione di evidenze morfologiche, sedimentarie e cronologiche. Il Quaternario 9, 387–392.
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Benvenuti, M., Carnicelli, S., Belluomini, G., Dainelli, N., Di Grazia, S., Ferrari, G.A., Iasio, C., Sagri, M., Ventra, D., Atnafu, Balemwald, Kebede, Seifu, 2002. The Ziway–Shala lake basin (main Ethiopian rift, Ethiopia): a revision of basin evolution with special reference to the Late Quaternary. Journal of African Earth Sciences 35, 247–269. Benvenuti, M., Carnicelli, S., Ferrari, G., Sagri, M., 2005. Depositional processes in latest Pleistocene and Holocene ephemeral streams of the Main Ethiopian Rift (Ethiopia). In: Blum, M.D., Marriott, S.B., Leclair, S.F. (Eds.), Fluvial Sedimentology VII, Special Publication, International Association of Sedimentologists, vol. 35. Blackwell, Oxford, UK, pp. 277–294. Bergner, A.G.N., Strecker, M.R., Trauth, M.H., Deino, A., Gasse, F., Blisniuk, P., Dühnforth, M., 2009. Tectonic and climatic control on evolution of rift lakes in the Central Kenya Rift, East Africa. Quaternary Science Reviews 28, 2804–2816. Biggs, J., Bastow, I.D., Keir, D., Lewi, E., 2011. Pulses of deformation reveal frequently recurring shallow magmatic activity beneath the Main Ethiopian Rift. Geochemistry, Geophysics, Geosystems 12, Q0AB10. http://dx.doi.org/ 10.1029/2011GC003662. Bilham, R., Bendick, R., Larson, K., Mohr, P., Braun, J., Tesfaye, S., Asfaw, L., 1999. Secular and tidal strain across the main Ethiopian rift. Geophysical Research Letters 26, 2789–2792. Boccaletti, M., Bonini, M., Mazzuoli, R., Abebe, B., Piccardi, L., Tortorici, L., 1998. Quaternary oblique extensional tectonics in the Ethiopian Rift (Horn of Africa). Tectonophysics 287, 97–116. Bonini, M., Corti, G., Innocenti, F., Manetti, P., Mazzarini, F., Abebe, T., Pecskay, Z., 2005. Evolution of the Main Ethiopian Rift in the frame of Afar and Kenya rifts propagation. Tectonics 24, TC1007. http://dx.doi.org/10.1029/2004TC001680. Chernet, T., 2011. Geology and hydrothermal resources in the northern Lake Abaya area (Ethiopia). Journal of African Earth Sciences 61, 129–141. Chernet, T., Hart, W.K., Aronson, J.L., Walter, R.C., 1998. New age constraints on the timing of volcanism and tectonism in the northern Main Ethiopian Riftsouthern Afar transition zone (Ethiopia). Journal of Volcanology and Geothermal Research 80, 267–280. Coltorti, M., Corbo, L., Sacchi, G., 2002. New evidence for the Late Pleistocene and Holocene climatic changes in the Lake Region. In: Dramis, F. (Ed.), Proceedings of the Symposium: IAG International Geomorphologist Association. ‘Climate Changes, Active Tectonics and Related Geomorpologic Effects in High Mountain Belts and Plateaux’, Addis Ababa, 9–10.12.2002, pp. 30–35. Corti, G., 2009. Continental rift evolution: from rift initiation to incipient break-up in the Main Ethiopian Rift, East Africa. Earth Science Reviews 96, 1–53. Dainelli, N., Benvenuti, M, Sagri, M., 2001. Geological Map of the Ziway–Shala Lakes Basin (Ethiopia), Scale 1:250,000. DB-Map Firenze. Ebinger, C.J., Casey, M., 2001. Continental breakup in magmatic provinces: an Ethiopian example. Geology 29, 527–530. Ebinger, G.J., Yemane, T., Woldegabriel, G., Aronson, J.L., Walter, R.C., 1993. Late Eocene–Recent volcanism and faulting in the Southern Main Ethiopian Rift. Journal of Geological Society of London 150, 99–108. Gasse, F., Street, F.A., 1978. Late Quaternary lake-level fluctuations and environments of the Northern Rift Valley and Afar region (Ethiopia and Djibuti). Palaeogeography, Palaeclimatology and Palaeoecology 24, 279–325. Gasse, F., Van Campo, E., 1994. Abrupt post-glacial climate events in West Asia and North Africa monsoon domains. Earth and Planetary Sciences Letters 126, 435– 456. Gianelli, G., Teklemariam, M., 1993. Water-rock interaction processes in the AlutoLangano Geothermal field (Ethiopia). Journal of Volcanic and Geothermal Researches 56, 429–445. Gillespie, R., Street-Perrot, A.F., Switsur, R., 1983. Post-glacial arid episodes in Ethiopia have implications for climate prediction. Nature 306, 680–683. Hoefs, J., 1997. Stable Isotope Geochemistry, fourth ed. Springer, Verlag, Berlin, Germany, p. 201. Hotchkiss, E.R., Hall, R.O., 2010. Linking calcification by exotic snails to stream inorganic carbon cycling. Oecologia 163, 235–245. ISSC – International Subcommission on Stratigraphic Classification, 1994. A Guide to Stratigraphic Classification, Terminology, and Procedure, second ed. IUGS, Boulder. Laury, R.L., Albritton, C.C., 1975. Geology of the Middle Stone Age archaeological sites in the Main Ethiopian Rift Valley. Geological Society of America Bulletin 86, 999–1011. Le Turdu, C., Tiercelin, J.J., Gibert, E., Travi, Y., Lezzar, K., Richert, J., Massault, M., Gasse, F., Bonnefille, R., Decobert, M., Gensous, B., Jeudy, V., Tamrat, E., Mohamed, M.U., Martens, K., Balemwal, A., Chernet, T., Williamson, D., Taieb, M., 1999. The Ziway–Shala lake basin system, Main Ethiopian Rift: influence of volcanism, tectonics, and climatic forcing on basin formation and sedimentation. Palaeogeography, Palaeclimatology and Palaeoecology 150, 135–177. McCrea, J.M., 1950. On the isotopic chemistry of carbonates and a paleotemperature scale. The Journal of Chemical Physics 186, 849–857. Mohr, P.A., 1962. The Ethiopian Rift System. Bullettin of the Geophysical Observatory Addis Ababa 5, 33–62. Mohr, P., Zanettin, B., 1988. The Ethiopian flood basalt province. In: Macdougall, J.D. (Ed.), Continental Flood Basalts. Kluwer Academic, Dordrecht, pp. 63–110. Ohmoto, H., Goldhaber, M.B., 1997. Sulfur and carbon isotopes. In: Barnes, H.L. (Eds.), Geochemistry of Hydrothermal Ore Deposits. John Wiley & Sons, pp. 517–611. Pizzi, A., Coltorti, M., Bekele, A., Disperati, L., Sacchi, G., Salvini, R., 2006. The Wonji fault belt (Main Ethiopian Rift): structural and geomorphological constraints and GPS monitoring. 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Rollinson, H.R., 1993. Using Geochemical Data: Evaluation, Presentation, Interpretation. Longman, UK, 352pp. Sagri, M., Bartolini, C., Billi, P., Ferrari, G., Benvenuti, M., Carnicelli, S., Barbano, F., 2008. Latest Pleistocene and Holocene river network evolution in the Ethiopian Lakes Region. Geomorphology 94, 79–97. Stamps, D.S., Calais, E., Saria, E., Hartnady, C., Nocquet, J.M., Ebinger, C.J., Fernandes, R.M., 2008. A kinematic model for the East African rift. Geophysical Research Letters 35, L05304.
Street, F.A., 1979. Late Quaternary Lakes in the Ziway–Shala Basin, Southern Ethiopia. Ph.D. Thesis, Univ. Cambridge, 457pp. Street-Perrott, F.A., Perrott, R.A., 1990. Abrupt climate fluctuations in the tropics: the influence of Atlantic Ocean circulation. Nature 343, 607–612. WoldeGabriel, G., Aronson, J.L., Walter, R.C., 1990. Geology, geochronology, and rift basin development in the central sector of the Main Ethiopia Rift. Geological Society of America Bulletin 102, 439–458.
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Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci
Holocene lacustrine fluctuations and deep CO2 degassing in the northeastern Lake Langano Basin (Main Ethiopian Rift) Marco Benvenuti a,b,⇑, Marco Bonini b, Franco Tassi a, Giacomo Corti b, Federico Sani a,b, Andrea Agostini a, Piero Manetti a,b, Orlando Vaselli a a b
Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira 4, 50121 Firenze, Italy C.N.R, Istituto di Geoscienze e Georisorse, UOS Firenze, Via G. La Pira 4, 50121 Firenze, Italy
a r t i c l e
i n f o
Article history: Received 24 October 2011 Received in revised form 14 August 2012 Accepted 2 September 2012 Available online 20 September 2012 Keywords: Main Ethiopian Rift Holocene Lacustrine fluctuations Carbon geochemistry Active tectonics
a b s t r a c t This work reports the results of an integrated investigation on Holocene faulted deposits exposed on the northeastern Lake Langano in the central sector of the Main Ethiopian Rift (MER). The Lake Langano is part of a closed basin and thus it is highly sensitive to climate fluctuations. The present study explored the foot of the Haroresa escarpment, where coarse-grained slope deposits interbedded with thin lake shore shell-rich coarse sands are well exposed. Stratigraphical analysis of these deposits, integrated with radiocarbon dating carried out on lacustrine gastropod Melanoides tubercolata shells, points to significant lake level fluctuations forced mainly by climate oscillations. These have been interplaying during the early and middle Holocene with a structurally-controlled threshold separating an embayment of the Lake Langano to the south, from the small perched Lake Haro Bu-a basin to the north. Significant differences in the 13C/12C isotopic ratio have been identified in the M. tubercolata shells collected on the opposite sides of such a threshold. The time-space variations of the isotopic signature of the shells are referred to the mutual relationships between the two main different CO2 sources (i.e., microbial activity and deep mantle degassing) dissolved in the lake water. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
2. Hydrologic evolution of the Lakes Region
Researches on the Late Pleistocene–Holocene evolution of the Lakes Region in the Main Ethiopian Rift (Ethiopia), have concurred to update regional models of hydrologic responses to global climatic changes, providing a reference for the late Quaternary palaeoclimatology of the tropics (Street-Perrott and Perrott, 1990; Gasse and Van Campo, 1994). Along with such a palaeoclimatic relevance, the MER is also a zone characterized by active tectonic processes and intense hydrothermal activity (Gianelli and Teklemariam, 1993; Chernet, 2011). In such a dynamic setting, climatic, tectonic and magmatic-related processes may have exerted a concurrent and complex control on the Late Quaternary geomorphic and depositional dynamics. We focus on the northern coast of Lake Langano where lacustrine shells-bearing deposits and their radiogenic and stable carbon isotopic signature record Holocene, climate-driven, lake level fluctuations in the frame of a local active tectonic setting.
The Lakes Region includes lakes Ziway, Langano, Abiyata and Shala that developed since the Late Pleistocene (Street, 1979; Benvenuti et al., 2002) within the fault-controlled Main Ethiopian Rift depression (Fig. 1). Classically, the Holocene Ziway–Shala lake fluctuations (Street, 1979; Gasse and Street, 1978; Gillespie et al., 1983; Alessio et al., 1996; Fig. 2) have been considered a suitable proxy for the switching off and on of the monsoonal regime, as a regional response to global cooling and warming trends (StreetPerrott and Perrott, 1990; Gasse and Van Campo, 1994). Between 10 and 5 ky (conventional 14C ages), the lakes experienced an overall high-level status abruptly interrupted by short arid pulses. The early-middle Holocene (5–10 ky) wetter period coincides in the subarid tropics with a climatic optimum attesting to a relatively stable monsoonal circulation over East Africa. Short-lived arid pulses are related to a weakened monsoonal circulation which, in some cases, correlated quite well with global cooling events (Benvenuti et al., 2002). Since 5 ky to Present, the lakes progressively reduced in size due to increasing aridity. The progressive hydrological deficit over the Lakes Region during the late Holocene (post5 ky) may be related to increasing warming over the last 1000 years. Despite the evolution of the Ziway–Shala lacustrine system has been generally referred to a dominant climatic control,
⇑ Corresponding author at: Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira 4, 50121 Firenze, Italy. Tel.: +39 0552757516; fax: +39 055218628. E-mail address: [email protected]fi.it (M. Benvenuti). 1464-343X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jafrearsci.2012.09.001
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Fig. 1. Regional setting of the Lakes Region of the Main Ethiopian Rift. (a) Schematic tectonics setting of the East African Rift System; MER: Main Ethiopian Rift. (b) Detail of the Lakes Region in the central MER, showing the main Late Pleistocene–Holocene depositional units and their volcanic bedrock (after Benvenuti et al. (2002)). HRFS: Haroresa Rhomboid Fault System. Stars and related numbers refer to the location of lacustrine shell samples in Table 1.
120
height of overflow channel ?
?
Pleistocene, despite the early-middle Holocene climate was wetter than the last glacial (Benvenuti et al., 2002; Sagri et al., 2008).
?
Meters above the SD
3. Tectonic setting
SD (1558 m asl)
80
?
40
0 14
? 10
6
Age (C
14
2
0
kyr BP)
Fig. 2. Lake level fluctuations in the Lakes Regions (after Gillespie et al. (1983)). SD: Lake Shala datum 1558 m. a.s.l (1970s gauge data).
the tectonic setting of an active continental rift has played a significant role in controlling the surface hydrology as suggested for other lakes within the East African Rift System (e.g., Bergner et al., 2009). Around the Pleistocene–Holocene boundary, tectonically-driven stream piracy cut down a significant water budget to the lakes that was supplied by the northern catchments of paleo-Awash and paleo-Mojo rivers (Sagri et al., 2008). This determined a lake system less extended in the Holocene than in the Late
The Main Ethiopian Rift (MER) is part of the largest East African Rift System, and extends between the Afar depression to the North and the Kenya Rift to the South (Fig. 1a). The MER is characterized by a roughly NE-trending rift valley striking close to N–S in its southern sector which has been developing discontinuously since the Late Miocene in response to the extensional stress produced by Nubia–Somalia movement (e.g., Corti, 2009). The resulting fault pattern consists of two main fault systems: (1) a border fault set, and (2) a set of internal faults commonly referred to as Wonji Fault Belt (WFB; Mohr, 1962) that are best expressed in the northern MER. In the central MER, the border faults formed at 6–8 Ma (WoldeGabriel et al., 1990; Bonini et al., 2005), while the WFB faults started developing since about 2 Ma (Boccaletti et al., 1998; Ebinger and Casey, 2001), and are still in an incipient stage (Agostini et al., 2011). By controlling the rift floor morphology, the WFB activity gave rise to an interplay between climatic-forced lake level fluctuations and tectonics (e.g., Benvenuti et al., 2002). The northeastern Lake Langano, which is the focus of this paper, is adjacent to the eastern rift margin, which is typically characterized by a peculiar rhomboidal arrangement of faults with characteristic zigzag geometry, referred to as Haroresa Rhomboid Fault System (e.g., Le Turdu et al., 1999). This system essentially affects the Pliocene rift floor ignimbrites and locally the Middle Pleistocene–Holocene colluvial–alluvial deposits and pyroclastics (Dainelli et al., 2001). Such a recent fault activity probably
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controlled the emplacement and circulation of hydrothermal fluids associated with the Aluto-Langano geothermal field (Gianelli and Teklemariam, 1993), where volcano-tectonic activity is also documented by satellite imagery analysis (Biggs et al., 2011). The tectonic evolution of the MER was preceded by intense volcanic activity, which started with the eruption of Oligocene (30– 35 Ma) voluminous flood basalts (e.g., Mohr and Zanettin, 1988; Ebinger et al., 1993; WoldeGabriel et al., 1990; Chernet et al., 1998; Fig. 1). Subsequent magmatism was marked by a second episode of pre-rift flood basalts at 8–12 Ma (Abebe et al., 2005; Bonini et al., 2005), followed by the eruption of widespread rhyolitic ignimbrites (Rift Floor ignimbrites or Nazret Pyroclastic Rocks of Abebe et al., 2005; Fig. 3) that marked the onset of rift activity in the Late Miocene. After this widespread Mio-Pliocene volcanism, the Quaternary magmatic activity mostly localized in the rift floor, with products showing a typical bimodal composition given by alternating basaltic (e.g., Wonji basaltic activity; Wonji Basalts of Abebe et al., 2005; Fig. 3) and rhyolitic (e.g., Aluto-Bericcio products; Bora-Bericha Rhyolites of Abebe et al., 2005; Fig. 3) activity mostly associated with the WFB faulting.
4. Stratigraphic and paleogeographic setting of the Late Quaternary continental deposits Following to the seminal studies of Street (1979), Gasse and Street (1978) and Gillespie et al. (1983), in the last decade a stratigraphic revision of the Late Quaternary succession, covering Pliocene–early Quaternary volcanic and volcaniclastic deposits (Abebe et al., 2005), allowed to trace four unconformity-bounded units (ISSC, 1994; synthems 1–4) over the Lakes Region that record major stages in the lake evolution (Fig. 1b; Benvenuti et al., 2002). In the northern Lake Langano area, the Late Pleistocene–Holocene
3
lacustrine succession overlies reddish alluvial–colluvial conglomerates, sandstones and mudstones ascribed to the latest Middle Pleistocene (Coltorti et al., 2002). The synthems composing the Late Quaternary deposits are briefly described from bottom to top (see Fig. 1). Synthem 1 consists of colluvial, fluvio-deltaic, lacustrine gravels, sands, and muds, diatomites and volcaniclastic materials, deposited during the last glacial period, ca. 100,000 to 22,000 yrs BP (Megalake phase; Benvenuti et al., 2002; Fig. 1). This latter lake system expanded during wetter interstadials and contracted during the drier stadials. Synthem 2 is dominated by alluvial–colluvial and volcaniclastic deposits accumulated during the last full glacial and late glacial, ca. 22,000–10,000 yrs BP when arid conditions brought to severe shrinking of the lakes (Reduced Lake Phase, Benvenuti et al., 2002; Fig. 1). Synthem 3 consists of colluvial, fluvio-deltaic, lacustrine gravels, sands, and muds, diatomites, and volcaniclastic materials recording the early-middle Holocene, ca. 10,000– 5000 yrs BP (Macrolake phase of Benvenuti et al. (2002); Fig. 1). This lake system, smaller than the former Megalake, underwent frequent level fluctuations (Benvenuti et al., 2002; Benvenuti et al., 2005) with a maximum lake surface elevation of about 1670 m above sea level (Street, 1979; Laury and Albritton, 1975; Benvenuti et al., 2002). Synthem 4 consists of colluvial, fluvial, deltaic and lacustrine sediments accumulated over the last 5000 years when Macrolake progressively split into the present four lakes (Separated Lakes Phase of Benvenuti et al. (2002); Fig. 1).
5. The Haroresa road cross-section In February 2007, a shallow ditch excavated along the steep slope delimiting the eastern escarpment of a faulted block on the northeastern coast of the Lake Langano (Haroresa escarpment;
Fig. 3. Geological map of the northern Lake Langano basin (adapted from Dainelli et al. (2001)).
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Figs. 3, 4a and 5) allowed the direct observation and sampling of a few meter thick clastic succession recording a mix of slope and lacustrine depositional settings (Fig. 3). The excavation also brought to light a fault zone through-going these deposits, which was also surveyed in February 2009 (Fig. 5a and b). 5.1. Structural setting The northeastern corner of the Lake Langano lies within the Haroresa Rhomboid Fault System (HRFS), which has resulted from the intersection between two fault trends: a dominant NNE to NEtrending fault set, and a minor N-to-NNW-trending fault set. These fault systems gave rise to horst, graben and half-graben structures mainly trending around NNE to NE, with the principal river courses flowing within the elongated depressions. The northeastern Lake Langano corner is influenced by both HRFS fault sets (Fig. 3). In the Haroresa-Haro Bu-a area, the dominant NNE- to NE-trending and southeast-dipping normal faults produce several major northwest-dipping tilted blocks forming a series of 0.5- to 3-km-wide half-grabens (Figs. 3 and 4b; Le Turdu et al., 1999). These depressions are filled by Quaternary sediments or are occupied by small lakes or swamps onlapping onto the major bounding fault scarps, such as for the Haroresa and Haro Bu-a escarpments (Figs. 3 and 4). The fault scarps are 50–150 m-high, generally steep (>60°), and exhibit a fresh morphology (Fig. 4). The Lake Haro Bu-a is located in the overlapping zone between two NNE-trending faults that join southwestward into a single fault, which is magnificently exposed by the Haroresa road crosssection. The fault separates the Pliocene rift floor ignimbrite on the fault footwall from the clastic succession made of coarse grained slope debris and lacustrine deposits bearing abundant Melanoides tubercolata shells (see below). The fault zone is characterized by closely spaced shear planes affecting the Holocene clastic succession (Fig. 5a and b), thereby demonstrating ongoing fault
activity that is also coherent with its prominent morpho-tectonic signature (Fig. 4). The fault planes record evident striations that allowed the measurements of a number of good quality fault-slip data. The striations show a main pure to slight oblique-slip kinematics that is consistent with a local ca. E–W-trending Holocene extension direction (Fig. 5a). In agreement with these results, fault-slip data collected at sites distributed on this sector of the eastern rift margin indicate an overall N100°E-directed Quaternary extension (Agostini et al., 2011). These data accord well with previous paleostress determinations carried out on the same rift margin (Bonini et al., 2005; Pizzi et al., 2006) as well as with the results of geodetic analyses (Bilham et al., 1999; Stamps et al., 2008). 5.2. Geomorphology and stratigraphy The outcrop is exposed for some hundred meters over the morpho-tectonic landscape related to the HRFS (Figs. 3 and 4). The studied sections are located just at the transition between the isolated Lake Haro-Bu-a and the Langano embayment (Fig. 4). The Haroresa escarpment consists of three distinct sectors. The midupper slope is made of plane-bedded Rift Floor Ignimbrites, which determine a stair-step morphology of the slope. In the mid-lower portion two differently inclined surfaces occur. The steeper and higher one subtends boulder-sized colluvial deposits delimited on top by a flat surface representing a lacustrine terrace (Fig. 4a). On the basis of available elevation data, the current altitude of this terrace stands between 1660 and 1680 m above sea level, which falls in the range of the maximum early-middle Holocene Macrolake level. The lower slope surface, cut by the road excavations, is less inclined and exposes a mix of coarse and fine-grained debris, which represents the target of more detailed observations. Stratigraphic description and sediment sampling for radiocarbon analysis were carried out on two distinct outcrops (sites 1 and 2)
Fig. 4. (a) View of the Haroresa fault escarpment. The white boxes indicate the stratigraphic sections 1 and 2 described in the text. (b) Lateral view of the northern segment of the Haroresa fault escarpment downthrowning Lake Haro Bu-a and tilting the lacustrine deposits. The open black arrow indicates the Haroresa normal fault. The open white arrows indicate the fault-bounded tilted blocks. The tip of white triangles indicate the base of the morphologic fault scarp. Photo view points are indicated in Fig. 3.
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Fig. 5. Structural and stratigraphic features along the Haroresa escarpment. (a) Major fault plane (indicated by white arrows); the stereonet on the top-right handside shows the collected fault slip data (Wulff net lower hemisphere; after Agostini et al. (2011). (b) Detail of the fault zone with a series of parallel shear planes (indicated by white arrows) affecting the slope debris and Holocene lacustrine sediments. (c) Slope deposits with minor lake shore shell-rich pebbly sands exposed on the fault hanging-wall (persons for scale); 1a and 1b indicate the samples collected for radiocarbon dating (see Table 1). (d) Close-up of the shore pebbly sand-bearing shells (gastropod M. tubercolata) collected at site 1a.
separated by a saddle separating the Langano embayment from the Lake Haro-Bu-a (see location in Fig. 4a).
sediments during lake transgressions and gravity transport during regressive stages.
5.2.1. Site 1 The clastic succession exposed at site 1 on the hangingwall of the Haroresa normal fault (visible in Fig. 5a), consists of about 5– 6 m thick blocky angular clasts deriving from the Rift Floor Ignimbrites, and subordinate pebbly sands (Fig. 5b and c). In the lower part of the outcrop, pebbly sands rich in freshwater mollusk shells, dominated by the gastropod M. tubercolata (Fig. 5c and d), attest to a lacustrine nearshore environment. These deposits are overlain by crudely layered cobble-boulder sized gravel dispersed in a variable amount of finer matrix. Clast composition is monogenic and made of the Rift Floor Ignimbrites. Textures of these deposits ranges from clast- to matrix-supported (Fig. 5c). On the whole, these deposits are referred to debris fall and debris flow that occurred along the steep Haroresa escarpment. In specific clast-supported beds, the boulders are coated by a few millimeter-thick withish massive to laminated carbonate crust (Fig. 5c). Two samples were collected for radiocarbon analysis (Fig. 5c). Sample 1a includes several shell specimens of M. tubercolata, sample 1b is a single clast whose carbonate coating was separated in laboratory (Table 1). The depositional setting at site 1 oscillated between a lacustrine rocky coast confining the Langano embayment during lake high-stand and a subaerial slope during lake regression. Therefore, the operating depositional processes were the biogenic carbonate precipitation and wave reworking of the clastic
5.2.2. Site 2 The ditch excavated on the upslope side of the road at site 2, a few tens of meters from site 1 (Fig. 4a), shows a quite different succession with respect to that of site 1. Despite its limited thickness, this succession is characterized by a more articulated architecture (Fig. 6a). In particular, the basal deposit is a massive pebbly–cobbly gravels with abundant sandy–silty matrix (Fig. 6b). The basal deposit is overlain by pebbly sands with abundant mollusk shells dominated by M. tubercolata indicating a paleoshore at the southern end of the Lake Haro Bu-a. A thin fine-grained ash bed rests over the pebbly sand deposit, and it is unconformably overlain (Fig. 6a and b), by a deposit consisting of pebbly sands, reworked ash and abundant M. tubercolata shells. Both Melanoides-bearing beds onlap basal massive gravels, and outline the southward termination of the Haro Bu-a lacustrine area (Fig. 6c). Such a marginal lake succession is overlain unconformable by a thick matrix-rich gravels sustaining a modern dark stony soil (Fig. 6a and c). Two samples (2a and 2b) of several specimens of Melanoides shells have been collected for radiocarbon dating, respectively below and above the angular unconformity separating the two layers of Melanoides-bearing sands (Fig. 6b; Table 1). Depositional processes were dominated by gravel transport in short creeks draining the slopes during the regressive stages, and by wave reworking in a nearshore environment during high-lake
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Table 1 13 12 C/ C isotopic ratios and conventional ages of samples of Melanoides and/or mixed lacustrine shells collected from different portions of the Ziway–Shala basin (location in Fig. 1b). Radiocarbon dating (materials measured by the standard radiometric technique; see http://www.radiocarbon.com for details) has been carried out in this study (samples 1a, 1b, 2a, 2b) and during 1995–1998 (samples 311) by both the Beta Analytic Radiocarbon Dating Laboratory (Miami, Florida, USA) and CNR-Istituto per le Tecnologie Applicate ai Beni Culturali (Rome, Italy). The radiocarbon ages of samples 3–11 are published in Benvenuti et al. (2002), whereas the correspondent 13C/12C isotopic ratios are unpublished. Sample
Laboratory code
13
C/12C
Conventional age
Latitude
Longitude
1a 1b 2a 2b 3 4 5 6 7 8 9 10 11
Beta 231103 Beta 263657 Beta 231101 Beta 231102 Beta 114024 R-2425 R-2426 R-2441 R-2444 Beta 114023 Beta 098943 Beta 114021 Beta 114022
3.3 o/oo +3.8 o/oo 5.0 o/oo +0.2 o/oo 1.7 o/oo 3.3 o/oo 2.5 o/oo 2.8 o/oo 3.0 o/oo 4.3 o/oo 3.1 o/oo 2.8 o/oo 2.3 o/oo
5840 ± 60 BP 7190 ± 60 BP 9730 ± 60 BP 6350 ± 50 BP 5820 ± 60 BP 5992 ± 43 BP 7642 ± 43 BP 10,347 ± 79 BP 11,590 ± 203 BP 27,040 ± 260 BP 30,440 ± 160 BP 39,170 ± 760 BP 47,470 ± 1600 BP
7°400 22.9400 N 7°400 22.9400 N 7°400 26.6900 N 7°400 26.6900 N 7°280 10.4400 N 7°470 28.3400 N 7°480 44.0600 N 7°480 44.0600 N 7°480 44.0600 N 7°470 28.3400 N 7°570 45.2300 N 7°570 45.2300 N 7°470 28.3400 N
38°480 37.8700 E 38°480 37.8700 E 38°480 36.9400 E 38°480 36.9400 E 38°380 51.4700 E 38°410 46.4700 E 38°410 39.4400 E 38°410 39.4400 E 38°410 39.4400 E 38°410 46.4700 E 38°390 50.6700 E 38°390 50.6700 E 38°410 46.4700 E
levels. The thin ash beds record syndepositional pyroclastic fall in the Haro Bu-a basin attesting to volcanic activity in the surrounding. 5.3. Carbon radiogenic and stable isotope geochemistry Samples 1a–b and 2a–b were analyzed for the determination of the radiocarbon ages at the Beta Analytic Laboratories according to AMS and standard techniques (see http://www.radiocarbon.com/ for analytical details). These shell samples, together with lacustrine shells collected from nine sites located in different portions of the broader Ziway–Shala basin – i.e., along the shoreline of Lake Shala (sample 3), west of Alutu Mt. (samples 4, 5, 6, 7, 8, and 11) and west of Lake Ziway (samples 9 and 10) (Fig. 1b) were analyzed for the determination of the 13C/12C ratios following the procedure proposed by McCrea (1950). The 14C data (Table 1) indicate that the studied sections are fully Holocene in age, allowing the correlation of the local stratigraphy to synthems 3 and 4 established in the whole Ziway–Shala basin. Ages of samples 1a and 1b outline a marked stratigraphic inconsistency, which could be referred to the following evolution: (i) algal carbonate coating of upslope colluvial deposits during the maximum Megalake highstand at about 1670 m a.s.l. (marked by the lacustrine terrace in Fig. 4a), and (ii) downslope re-deposition of the coated clasts (i.e., sample 1b) during regressive phases postdating the deposition of shoreline deposits (i.e., sample 1a). Samples 2a–b yielded radiocarbon ages consistent with their mutual stratigraphic position. In spite of a very close 14C age, samples 1a and 2b have different 13C/12C ratios (3.3 and +0.2 permil V-PDB, respectively; Table 1). The 13C/12C ratios of samples 3–11, whose radiocarbon age range from 47,000 to 5000 years BP (Benvenuti et al., 2002), define rather similar negative values ranging from 1.7 to 4.3 permil V-PDB (Table 1). 6. Discussion The stratigraphic analysis confirmed the role of environmental changes as mainly driven by climate variations. Late Quaternary tectonics and volcanism in the MER modulated such a climatic control by forcing geometric changes in the basin and determining a significant variation in its areal extension, as manifested by the transition from the late Pleistocene Megalake system to the early-mid Holocene Macrolake system (Fig. 1). Even assuming slip-rates up to 1 mm/yr for the active faults, the resulting uplift rates are not comparable with the meters-scale climatic-forced lake level fluctuations occurring at least one order of magnitude faster. Nevertheless, the studied sections provide interesting elements for integrating sedimentary, tectonic and geochemical lines
of evidence of the Holocene evolution of both the Langano embayment and the adjacent Haro Bu-a basin.
6.1. Stratigraphic and tectonic constraints on the palaeohydrologic dynamics of the Langano embayment-Haro Bu-a basin The road-cut from site 1 to site 2 crosses the modern fault-controlled threshold separating the Lake Langano from the Haro Bu-a basin, which also acted as a significant morpho-structural, hydrological and geochemical boundary during the great part of the Holocene. Such a threshold is the expression of an active tectonic setting of the HRFS (Le Turdu et al., 1999). The fault plane brought to light by excavation at site 1, documents a late Holocene (post 6200 14C years BP; sample 1a) activity of the fault system giving rise to the Haroresa escarpment (Agostini et al., 2011). From a stratigraphic point of view, the sections exposed at sites 1 and 2 reflect at a local scale the general evolution of the Holocene Ziway–Shala lakes basin (Benvenuti et al., 2002): early-mid Holocene synthem 3 and late Holocene synthem 4 left clear traces as marginal lacustrine and slope deposits, respectively (Figs. 5 and 6). As regards the synthem 3 deposits, the comparison between the elevation/chronology of the shell beds 1a, 2a, and 2b and the overall Holocene lake fluctuations (Gillespie et al., 1983), locate the dated sediment during stages of rising or high-lake levels (Fig. 2). The reconstructed lake level fluctuations across the saddle are schematically illustrated in Fig. 7. The available topographic data allows to place the present threshold separating the Langano embayment from the Haro Bu-a lake around 1650 m. a.s.l., with the shell samples (1a, 2a and 2b) being collected a few meters below the threshold (Figs. 4a and 7). Both the sedimentary features observed in the sampled shell beds and the stratigraphic onlap at the site 2 section testify to the deposition in nearshore environments. These latter pertained to the Lake Langano (sample 1a) and Lake Haro Bu-a (samples 2a–2b), which were thus separated at about 9730 14C years BP (sample 2a; Fig. 7a) and around 6200–6300 14C years BP (samples 1a–2b; Fig. 7c; Table 1). A higher lake level is indirectly recorded by the 7190 14C years-old thin stromatolite coating some boulders in the slope deposits of synthem 4 at site 1 (sample 1b; Fig. 7b). The stromatolite evidences a high lacustrine stillstand that is morphologically recorded by a terrace cutting into the Haroresa escarpment at around 1670 m a.s.l,, and that likely corresponds to the maximum level of the early-mid Holocene Macrolake. In agreement with Gillespie et al. (1983), the Ziway–Shala lakes level was rather high at around 7000 14C years BP, and was possibly approaching its maximum level that reached between 6000 and 5000 14C years BP (Fig. 2). Under this hypothesis, the Langano embayment and the Lake Haro Bu-a may
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Fig. 6. (a) Stratigraphy of the site 2 section: early Holocene shore and slope deposits, light in color, are overlain by recent brownish matrix-rich boulders accumulated at the toe slope of the Haroresa escarpment facing the lake Haro Bu-a basin (see Fig 4). Black arrows outline the southward stratigraphic onlap of the upper Haro Bu-a shore deposits onto an ash bed topping the lower shore deposits and the underlying slope deposits. The rod is 1 meter long. (b) Detail of the area enclosed by the white box in (a) showing the stratigraphy described in the text together with the location of samples 2a and 2b, which consist of several specimens of M. tubercolata collected for radiocarbon dating (see Table 1). The thick black dashed lines indicate unconformities. (c) Pinch-out of the lake shore deposits toward the south, i.e., approaching the threshold separating the Langano embayment from the isolated lake Haro Bu-a (see Fig. 4a); the thick black dashed lines indicate unconformities.
have been joined about 7190 14C years ago (Fig. 7b). In hydrological terms, the story reconstructed from sites 1 and 2 stratigraphy tells about a dominant isolation of the Lake Haro Bu-a from the Langano embayment, interrupted by a short-lived connection during lacustrine highstand around the mid Holocene maximum lake level (Fig. 7). A NE-dipping angular unconformity separates the lower and the upper mollusk-bearing paleo-shore deposits (Fig. 6), which correspond to the two major Lake Haro Bu-a highstands at ca. 9730 14C years BP (sample 2a) and around 6200–6300 14C years BP (sample 2b; Table 1). Such a bottom lake tilting is consistent
with an event of tectonic subsidence affecting the fault-controlled Haro Bu-a basin. Both faulting evidence and the presence of ash fall deposits across the unconformity attest to an episode of volcanotectonic activity during early-middle Holocene. 6.2. Endogenic vs. exogenic isotopic signatures of Carbon in Melanoides shells The carbon geochemistry, derived from the dating of carbonate organic remains collected at sites 1 and 2, reveals an intriguing variation in the 13C/12C isotopic ratio of the M. tubercolata shells.
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Fig. 7. Schematic cartoon showing the reconstructed 3-step lake-level fluctuations of the Langano-Haro Bu-a threshold in relation to the supposed post9730 yr BP increase in mantle-derived CO2 supply. The position of dated samples is shown, together with the Conventional Radiocarbon Age and the 13C/12C ratio (in parenthesis). See text for details.
Owing to the different basin dimensions, distinct environmentalgeochemical conditions (e.g. water temperature and pH) could have been established within the Lakes Haro Bu-a and Langano. In particular, these conditions might have caused different carbon isotopic signatures of CO2, and, consequently, of bicarbonate involved in the cycle of the biotic calcification process of M. tubercolata, as described by the following reaction (Hotchkiss and Hall, 2010):
Ca2þ þ 2HCO3 ! CaCO3 þ H2 O þ CO2 Therefore, a variation of the 13C/12C ratios of CO2 dissolved in the Lake Haro Bu-a water may have indirectly caused a change of the 13 12 C/ C ratios of the M. tubercolata shells, which used bicarbonate ions to construct their exoskeletons. By reference to the lake water, the CO2 sourced from within a system, such as that produced by microbial activity, can be referred to as ‘endogenic’; the CO2 coming from outside the lake system, such as the mantle, is instead ‘exogenic’. It is worth mentioning that the 13C/12C value of mantle-related CO2 is typically heavier (from 7 to 3‰ V-PDB) than that of microbial CO2 (<20‰ VPDB) (Rollinson, 1993; Hoefs, 1997; Ohmoto and Goldhaber, 1997). Deep-originated CO2 has been likely discharged into the Lake Langano system through fluid emissions related to the strong volcano-tectonic and geothermal activity characterizing this sector of the MER (Gianelli and Teklemariam, 1993; Biggs et al., 2011).
The establishment of isolation phases of the Lake Haro Bu-a from the Lake Langano are revealed by the paleo-shore deposits at site 2 (Figs. 6 and 7). The shells collected in the two distinct paleo-shore layers show marked difference in the 13C/12C ratios (samples 2a and 2b; Fig. 6 and Table 1). This isotopic difference could be explained by variations in (i) environmental conditions (influencing biogenic CO2 production occurring within the lake) and/or (ii) hydrothermal activity (regulating the exogenic CO2 contribution). The shells contained in the older paleo-shore beds (sample 2a), deposited during an early phase of isolation, yield a negative 13 12 C/ C ratio (5‰ V-PDB) that correlates regionally with the negative values of the 13C/12C isotopic ratio determined on shells of samples 3–11 (Table 1) representative of the Mega- and Macrolake systems (Benvenuti et al., 2002, and unpublished data; Fig. 1). In this scenario, sample 2b is the only showing a positive isotopic ratio (13C/12C = +0.2), implying that the isolated Lake Haro Bu-a recorded a major change in the isotopic composition of the lake water between 9730 yr BP (sample 2a) and 6350 years BP (sample 2b). Such a positive 13C/12C ratio could manifest the input of relatively high amounts of isotopically heavy mantle-related CO2 into the system, which has strongly affected the carbon isotopic signature of carbon-bearing dissolved ions (mainly HCO3) of the small Lake Haro Bu-a. The larger Lake Langano would have in fact buffered any exogenic deep-input of 13C-rich CO2 because microbial activity represented the main (endogenic) CO2 source controlling the negative isotope ratio of the carbon-bearing ions of the lake and, consequently, of the shell composition (cf. the 13C/12C ra-
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tios of the roughly coeval samples 1a and 2b; Table 1). In other terms, the isolation of the Lake Haro Bu-a, in combination with the recorded variation in the 13C/12C ratio in the M. tubercolata shells collected within the Lake Haro Bu-a basin (samples 2a and 2b), have allowed the recognition of a significant variation in the tectono-magmatic activity in this rift portion during early-middle Holocene times. The effects of evaporative processes operating in the different sized Lakes Langano and Haro Bu-a cannot be invoked to explain the difference between the isotopic values of the samples 1a and 2b, because a very different carbon isotopic signature of shells has been recorded during the two distinct isolation events of the Lake Haro Bu-a (i.e., samples 2a and 2b; cf. Fig. 7a with 7c). As for as the 13C/12C ratio of stromatolites (sample 1b) is concerned, this cannot be directly compared with those of the M. tubercolata shells, because the carbon isotope fractionation related to the biocalcification processes of these two types of living organisms may be different. From the above discussion, we interpret the presented data as evidence that the input of deep exogenic CO2 into the geothermal system of the Lake Langano increased between 9730 years BP (sample 2a; Fig. 7a) and 6350 years BP (sample 2b; Fig. 7c). The network of rift faults described in the area has been shown to consist of several active structures, which thus represent the most likely candidate that accomplished such a supposed post9730 year BP transfer increase of mantle fluids to shallower levels. 7. Concluding remarks This integrated work has allowed to put forward the following main points: (1) Geochemical and radiocarbon dating of lacustrine shells, corroborated by detailed stratigraphic field analysis, have allowed reconstructing local variations in Lake Langano level during early-middle Holocene (5–10 ky), which accord with, and may refine, the reconstructed regional hydrologic fluctuations. (2) Local active faults may bear a two-fold importance by providing both (1) the topographic frame onto which the lake level fluctuations have been operating, and (2) potential deeply-rooted fault-controlled pathways exploited by ascending fluids. (3) Finally, the present work may offer a case history in which lake-level fluctuations modulated by climatic forcing combine with variations in the supply of CO2-rich fluids sourced from the mantle up-through active faults.
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