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Article

Opposite Hydrological Conditions between the Younger Dryas and the 8.2 ka Event Revealed by Stalagmite from Northwest Madagascar in East Africa

by
Pengzhen Duan
1,
Hanying Li
2,*,
Gayatri Kathayat
2,
Haiwei Zhang
2,
Youfeng Ning
2,
Guangyou Zhu
1 and
Hai Cheng
2,*
1
Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China
2
College of Global Environmental Change, Xi’an Jiaotong University, Xi’an 710049, China
*
Authors to whom correspondence should be addressed.
Minerals 2024, 14(4), 348; https://doi.org/10.3390/min14040348
Submission received: 8 January 2024 / Revised: 28 February 2024 / Accepted: 7 March 2024 / Published: 27 March 2024
(This article belongs to the Special Issue Stalagmite Geochemistry and Its Paleoenvironmental Implication)
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Abstract

:
As prominent abrupt climatic events during the last deglaciation and the early Holocene, the Younger Dryas (YD) and the 8.2 ka events have been intensely discussed to reveal the relationship between their phases and intensities, and their underlying mechanisms based on massive marine and terrestrial archives. However, the related paleoclimate records with sufficient resolution and/or precise age constraints from the Southern Hemisphere, especially East Africa, are relatively sparse, hindering our comprehensive understanding about the phases of these two events. Here, we provide a precisely dated record of an aragonite-calcite stalagmite covering 11.3–13.5 ka BP from northwest Madagascar to unravel the arid conditions during the YD, in contrast to the pluvial conditions in the 8.2 ka event that has been evidenced before. Changes in austral summer precipitation related to the Intertropical Convergence Zone (ITCZ) have always been interpreted to be the primary means of controlling regional rainfall amounts and thus the δ18O variations in stalagmite. However, ITCZ’s meridional migration alone is not enough to interpret the opposite hydroclimatic conditions during the YD and the 8.2 ka events in northwest Madagascar. The variation in convection intensity within the ITCZ combined with the rainfall dipole mode in East Africa, and the redistribution of the duration of the ITCZ’s presence at different latitudes might be responsible for this phenomenon. In addition, sea surface temperature could play a nonnegligible role.

1. Introduction

The Younger Dryas (YD) and the 8.2 ka events are the two most dramatic examples of abrupt climate events that occurred before and after entering the Holocene, and both of them have been captured by various global archives and investigated in terms of timing, structure, and underlying mechanisms [1,2,3,4,5,6,7,8,9,10,11,12]. The driving mechanisms of both events were associated with abrupt large volume freshwater discharge into the North Atlantic and consequent slowdown of Atlantic Meridional Overturning Circulation (AMOC) [13,14,15,16,17,18,19]. As a result, the temperature in the high latitudes of the Northern Hemisphere dramatically declined [1,2,20,21], the Asian monsoon intensity weakened [4,5,22,23], and the latitudinal location of the Intertropical Convergence Zone (ITCZ) was displaced southwards [3,7,24,25,26,27,28]. Furthermore, paleoclimate records from the Southern Hemisphere indicated a strengthened South American Summer Monsoon [5,29,30,31,32], a dry tropical East Africa [6,9,10,33,34,35,36], and humid conditions in southern East Africa [27,37,38,39] during the YD and the 8.2 ka events.
Although the YD and the 8.2 ka events show similar patterns/trends in their individual hemispheres [16,17,40,41,42,43], the duration and intensity of these two events are distinctly different. Under the background of a deglacial interval of the last Ice Age, the estimated fresh water injection triggering the YD was more than 10 times higher than the 8.2 ka event. The YD lasted for approximately 1300 years, commencing at approximately 12.85 ka BP (a thousand years before present time, where present = AD 1950) (e.g., [1]) with temperatures dropping by 4–6 °C in the circum-North Atlantic regions [44,45,46]. In comparison, the 8.2 ka event is a centennial abrupt climate perturbation inset into the early Holocene with modest cooling of 1–3 °C across large parts of the Northern Hemisphere, and this condition only lasted for 100–200 years [2,5,16,18,40]. According to the Greenland ice core δ18O [1] and stalagmite δ18O records from Dongge cave in China [4,5], the excursion amplitude of the YD is 2–4 times that of the 8.2 ka event.
Unlike South America, where both the YD and the 8.2 ka events were clearly revealed as being monsoon periods strengthened by stalagmites [5,29,30,31,32], the similarity and differences between these two events are yet confusing in tropical and southern East Africa. To the east of the Africa continent, stalagmites demonstrated that northwest Madagascar experienced wet conditions during the 8.2 ka event [47,48,49]. In the tropical East Africa continent, the YD was documented as a dry period in lake records, such as those for Lake Challa [10,11,50,51], Lake Tanganyika [9], Lake Malawi [7,33,52,53,54], and Lake Masoko [55,56,57], and marine sediment records from the coast of Tanzania [35] and Mozambique Channel [27,28,29], consistent with the observations of stalagmite from Mauritius [58]. However, except the records from Southwest Madagascar that cover the period of 12–22 ka BP [37], the paleoclimate reconstructions with sufficient resolution to recognize the YD are almost not available from lake [59] and other stalagmite [47,48,60,61,62] records in northwest Madagascar, either due to their low resolution or the absence of this period. Therefore, the relationship between these two events still remains obscure, and thus, further evidence, in particular covering both events in the same record, is needed to understand the behavior of the East Africa rain belt in response to various degrees of North Atlantic abrupt cooling events.
Located at the southern limit of the ITCZ, northwest Madagascar’s hydroclimatic condition is pretty sensitive to the climate variations. Therefore, this site is ideal for exploring the phase relationship between the YD and the 8.2 ka event in East Africa. Here, we use a stalagmite (ABC-1) collected from Anjohibe (‘big cave’ in Malagasy) in northwest Madagascar to reconstruct the hydroclimate history that covers the YD event with robust chronological control using disequilibrium U-series dating techniques. The YD record is thoroughly revealed for the first time in this region. With these two abrupt events that occurred in the background of the last deglaciation and the early Holocene, separately, we can investigate the possible mechanisms that controlled the hydrologic climate variations in Southeast Africa.

2. Materials and Methods

2.1. Climate and Cave Settings

The island of Madagascar is located in the southeast coast of Africa, with a length of 1650 km from 12° to 25° S in the longitudes of 43–51° E. A 1200 m high mountain, whose massifs exceed 2600 m, runs through the entire island from north to south [63]. Lying at the western Indian Ocean off the east coast of Africa and at the southern limit of the ITCZ, currently, Madagascar mainly receives rainfall from the ITCZ. Stalagmite ABC-1 was collected from Anjohibe (15.54° S, 46.89° E) in the Majunga region of northwest Madagascar (Figure 1). Anjohibe has more than 24 entrances, with 5.3 km of cave passages [60]. In the vicinity of Anjohibe, the hills are generally more than 40 m high in carbonates over 100 m thick with grades from pure limestone at the base through calcareous dolomites to pure dolomites at the top [63]. According to modern monitoring, the annual range of temperatures in the 5.3 km cave passages is from 24.5 to 26 °C [48]. For the most part, drips are only active during and shortly after the rainy season [64]. Vegetation in the region is characterized by savanna grasses with small patches of mesic forest in wetter areas [60] and other trees adapted to the long arid season and periodic fires [60,63,65].
Anjohibe’s climate in general belongs to the tropical savanna climate, namely that rainfall is highly seasonal in the region. Over 80% of the mean annual rainfall (~1160 mm) occurs in austral summer from December to March, and on average, 97.6% of the rainfall occurs from November to April [63]. The annual mean temperature in this area is 26.4 °C, with a mean monthly temperature variation of ~4 °C throughout the year. During austral summer, when the ITCZ moves southwards into the Mozambique Channel, reaching 15° S (Figure 1), northwest winds bring large amounts of rainfall into northwest Madagascar, whereas during austral winter, the onshore easterly wind causes orographic rainfall on the east coast of Madagascar and a rain shadow to the west. It was observed that the rainfall δ18O values are enriched during austral winter and depleted during summer, whereas the decrease in the rainfall δ18O values is related to increased rainfall amounts [48].
The total length of stalagmite ABC-1 is ~450 mm (Figure 2). The upper part (0–230 mm) covering 8.0–8.5 ka BP [49] and 0.55–2.0 ka BP [66] has been published. This study provides the records of the remaining section, corresponding to 11.30–13.40 ka BP.

2.2. 230Th Dating and Age Model

The stalagmite ABC-1 was cut with a thin blade along the growth axes and then polished. To determine the depositional age of the stalagmite, powder samples weighing 15–120 mg were drilled along the stalagmite bands for U-Th radiometric dating with a carbide dental burr. The dating work was performed at the Isotope Laboratory of Xi’an Jiaotong University, China. All measurements were made on Thermo-Finnigan Neptune multi-collector inductively coupled plasma mass spectrometers using recently improved techniques [68]. We followed standard chemistry procedures to separate U and Th, as described in Edwards et al. (1987) [69]. The isotope dilution method with a triple-spike 229Th-233U-236U was employed to correct for instrumental fractionation and to determine the U and Th isotopic ratios and concentrations. The instrumentation, standardization, and half-lives are reported in Cheng et al. (2013) [68]. Uncertainties in the U/Th isotopic data were calculated offline at the 2σ level, including corrections for blanks, multiplier dark noise, abundance sensitivity, and contents of the same nuclides in the spike solution. The stalagmite’s chronology was constructed with the MOD-AGE model [67] based on 12 age determinations. Due to the obvious growth hiatuses, the age model of the stalagmite ABC-1 record was constructed for every growing interval separately before combining them together (Figure 2).

2.3. Stable Isotope Analysis

Oxygen and carbon stable isotope ratios were measured using a Thermo-Finnigan MAT-253 mass spectrometer and Delta-Plus XP IRMS linked to a Kiel IV Carbonate Device at Xi’an Jiaotong University, China. Subsamples were extracted at intervals of 1 mm with a dental drill along the central growth axis except for the section from 115 to 198 mm, where we increased the sampling resolution to 0.2 mm. CO2 was liberated from carbonate by reacting with phosphoric acid (for which the concentration was ~103%) and measured with working CO2 standard gas, for which the values were calibrated with NBS18 and IAEA-603. The results are reported as the δ notion, the per mil deviation relative to the Vienna Pee Dee Belemnite (VPDB) standard (δ18O = [((18O/16O)sample/(18O/16O)standard − 1) × 1000]). The precision of the δ18O and δ13C values at the 1σ level was ~0.06‰ and 0.03‰, respectively.

2.4. Mineralogical Determinations and Trace Element Analysis

To determine the mineralogy (calcite/aragonite) of stalagmite ABC-1, we conducted X-ray diffraction (XRD) analyses on 7 subsamples using a SmartLab X-ray diffractometer at Xi’an Jiaotong University in China. Each subsample weighed ~10 mg and was drilled adjacent to the stable isotope transect. Briefly, the XRD peaks produced by constructive interference of a monochromatic beam of X-rays scattered at specific angles from each set of lattice planes in a sample were the fingerprint of periodic atomic arrangements in a given material. A comparison of these data with a standard database for X-ray powder diffraction patterns enables identification of the crystalline samples.
Trace element ratio measurements (Sr/Ca and Mg/Ca) were performed at Xi’an Jiaotong University using the Laser-Induced Breakdown Spectroscopy (LIBS) technique [70,71,72]. Each sampling point was exposed to 5 laser shots before the measurements were performed in order to pre-clean the surface. Analyses were conducted by pulsing the laser to measure trace elements on every point in 0.3 mm increments along the growth axis of ABC-1. The recorded spectra were the average of 20 laser pulses at each position. The spectral data were processed using an interface created in MATLAB software (MATLAB R2019a) [71].

3. Results

3.1. Mineralogy and Trace Element

The XRD results showed that the study section of stalagmite ABC-1 is composed of both calcite and aragonite (Figure 3a). The high-resolution LIBS trace element analysis (0.3 mm interval) combined with the mineralogy identification results revealed that the aragonite sections have several times higher Sr/Ca and lower Mg/Ca ratios than those for calcite sections, in accordance with the argument that Sr (Mg) was known to be (not) preferentially incorporated into aragonite lattices versus calcite lattices [66,73,74]. As such, the distinct transitions in the Sr/Ca (Mg/Ca) record are probably indicative of alternating layers of aragonite and calcite. Therefore, it is reasonable for us to use the Sr/Ca (Mg/Ca) ratio record (Figure 3b) to delineate mineralogy changes in ABC-1 [64,66,75]. The inferred mineralogy phase is given in Figure 3c.

3.2. Radiometric Results

The dating results of stalagmite ABC-1 are presented in Table 1. Its 238U concentration is highly variable due to the changing mineralogy. The corrected 230Th age shows that the lower part of the ABC-1 was formed between 13.4 and 11.3 ka BP, covering the latest part of the last deglacial period and the early Holocene. The ABC-1 record can be divided into three intervals to characterize the Late Allerød (before ~12.75 ka BP), the YD (from ~12.75 to 11.6 ka BP) interrupted by a short-term hiatus, and the early Holocene (from 11.6 to 11.3 ka BP).

3.3. Stable Isotope Results

It has been proved that there is a relative depletion of 18O and 13C in the calcite layers compared to aragonite layers because of the different isotopic fractionation coefficients of oxygen between the two phases [76,77]. This offset for δ18O has been previously quantified in synthetic aragonite and in situ mineralogical studies to be between 0.6 and 1.0‰ at 25 °C [76,78,79]. Here, we converted the aragonite stable isotope values into their calcite equivalents using corrections of −0.8‰ for δ18O and −1.7‰ for δ13C [48,64,78].
The final stable isotope records are shown in Figure 3. Both the oxygen and carbon isotope values are highly variable from −2.82 to −7.31‰ and from −11.15 to −4.77‰, with average values of −5.05‰ and −8.76‰, respectively. The oldest part of the ABC-1 record (13.0–13.2 ka BP) is characterized by relatively depleted values. During the period of ~12.7–13.0 ka BP, the δ18O and δ13C values did not exhibit a large variability. After ~12.7 ka BP, a slow increase is observed in the stable isotope values, which might be identified as the onset of the YD. It remains challenging to discuss the details of the YD event because of the hiatus from 12.11 to 11.83 ka BP. However, it is apparent from the isotope distance diagram (Figure 3) that the oxygen isotope values rose by 2.5‰ to reach a mean value of −1.5‰ during the YD, and the δ13C values obviously became slowly enriched after ~12.74 ka BP. The average δ18O value was −3.64‰ from 12.74 ka BP to 11.65 ka BP. At the end of the YD, the δ18O values decreased gradually between 11.8 and 11.64 ka BP and reached an average value of −4.5‰ during the early Holocene. The early Holocene is characterized by depleted δ18O and δ13C values and statistically correlated δ18O and δ13C (r = 0.69).

4. Discussion

Northwest Madagascar has a highly seasonal precipitation pattern dominated by orographic effects and a seasonal visit of the ITCZ. The austral summer rainfall related to the ITCZ accounts for more than 80% in one year and has significant impacts on stalagmite δ18O values [47,48]. Recent studies have interpreted increases (decreases) in the δ18O values of stalagmite as indicators of less (more) rainfall in northwest Madagascar [47,48,49,63,80]. This is in concert with the significantly negative correlation between the simulated precipitation δ18O in northwest Madagascar and the local rainfall amount during austral summer, as demonstrated in a previous study [49]. Indeed, this negative relation is normally observed in tropical regions with common convective precipitation [81,82]. Hence, it is reasonable to demonstrate that the δ18O variations in stalagmite ABC-1 mainly reflect austral summer rainfall amount changes in northwest Madagascar.
In addition, the alternate deposition of calcite and aragonite in stalagmite is linked to hydroclimatic conditions when it was formed. The precipitation of primary calcite is commonly associated with high rainfall interval or relatively wet condition [47,48,83,84,85], and the deposition of aragonite has been ascribed to an increase in drip-water Mg concentration, probably related to more evaporation and prior aragonite precipitation (PAP), which might correspond to seasonal dryness [65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85]. Frisia et al. (2002) [79] pointed out that the presence of aragonite in stalagmite indicated a reduction in the drip rate related to either dry climate conditions or local hydrology. A previous study in northwest Madagascar showed that the presence of calcite mineralogy corresponded to lighter δ18O and δ13C values [47,48], which is observed in our case as well. Therefore, the alternate presence of calcite and aragonite layers can be used as an indicator of wet or dry conditions.

4.1. Arid Younger Dryas in Northwest Madagascar

As the most severe North Atlantic abrupt cooling event during the past 13 ka, the YD shows indistinctive excursion in the δ18O record of stalagmite ABC-1 during 12.7–12.2 ka BP due to a rebound to lighter values at ~12.55 ka BP. Yet, after an interruption of growth hiatus from 12.2 to ~11.8 ka BP, the ending period of the YD (11.65–11.8 ka BP) was characterized by apparently heavier values compared to the stage of 11.65–11.3 ka BP. A comparison between in the Greenland ice core δ18O record and northwest Madagascar reconstruction shows excellent correspondence in the termination timing of the YD, which has also been revealed by stalagmite records from the Asian monsoon area. Intriguingly, it appears that δ13C reached their highest isotopic values during the YD (Figure 3), likely indicating a transition from C3- to C4-dominated vegetation coverage, an increasing residence time in the epikarst, and a higher rate of prior carbonate precipitation, before transitioning back to lighter values and C3 vegetation compositions following the termination of the YD. Such changes in stalagmite δ13C likely reflect the changing rainfall availability [61,64,86]. Moreover, the mineralogy transition from the consecutive calcite during the Bølling-Allerød (B/A) event to an interbedded aragonite layer at ~12.41 ka BP, and eventually aragonite, could also point to the deteriorative hydroclimatic conditions during the YD event. Therefore, dry conditions were most likely prevalent in northwest Madagascar during the YD.
This drought event was similarly documented by various archives throughout tropical Southeast Africa (Figure 1 and Figure 4), including multi-proxies derived from Lake Challa [10,11,87] and isotopic compositions of leaf wax from Lakes Tanganyika [9,10] and Malawi [33,51]. In particular, the Rufiji River discharge history reconstructed with elemental log-ratios of the marine sediment off Tanzania revealed arid conditions in East Africa north of 8–10° S [35]. Further eastward in the Indian Ocean, stalagmite records from Rodrigues Island (Mauritius) signified a drought event during the YD through heavier δ18O values [58].
In contrast, to the south, anti-phase hydroclimatic conditions were revealed. Some sites (Zambezi Delta and Lake Chilwa) showed stages with high precipitation inferred from increased discharge or high-stand lake levels [27,38,39,88] (Figure 4). In addition, lower stalagmite δ18O records from southwest Madagascar signified that more rainfall occurred in this region during the YD [37]. In central Africa, paleoclimate records from western Zambia and western Zimbabwe [89] and from central Kalahari [90] also pointed to wetter conditions in response to the YD [27].
Indeed, the hydroclimatic variability over tropical East Africa is remarkably dominated by the Norther Hemisphere climate signal during the last deglaciation. In addition to the YD, stalagmite records from the same cave reveal prominently positive δ18O excursions during the Heinrich Stadial 1 (HS1) [91], which was believed to result from the weakening/shutdown AMOC, consistent with the large-scale drought conditions in the entire Southeast Africa [92]. This suggests a similar climatic response over the study area to those of the HS1 and YD, both of which were triggered by large amounts of freshwater influx into the North Atlantic. Following the HS1 [91] and before the YD (this study), although the records are not consecutive based on this study and Ref. [91], a substantial negative rebound was documented, corresponding to a wet B/A period. This dry HS1 and YD and wet B/A record is identical with the expectation in Northern Hemisphere, but antiphase with the interhemispheric “see-saw” pattern on the millennial scale [30,93,94].
Minerals 14 00348 g004
Figure 4. Comparison of ABC-1 δ18O record (f) with paleoclimate records during the Younger Dryas. From top to bottom: (a) NGRIP ice core δ18O record [1]. (b) Log (Ti/Ca) ratio record from marine sediment core in the mouth of the Rufiji River [35]. (c) Lake Challa δD record of leaf wax [11]. (d) Lake Tanganyika δD record of leaf wax [9]. (e) Lake Malawi BSi MAR record [7]. (g) Patate Cave δ18O record, Mauritius [58]. (h) Core GeoB9307-3 from Zambezi Delta [27]. (i) Fe/K ratio of marine core (CD154-17-17K), Eastern Cape [95]. (j) Marine core (CD154-10-06P) elemental ratios of Fe/K, KwaZulu-Natal of South Africa [96]. Yellow shading bars denote prominent cooling interval of YD indicated by Greenland ice core. The site locations of (b–j) are indicated in Figure 1.
Figure 4. Comparison of ABC-1 δ18O record (f) with paleoclimate records during the Younger Dryas. From top to bottom: (a) NGRIP ice core δ18O record [1]. (b) Log (Ti/Ca) ratio record from marine sediment core in the mouth of the Rufiji River [35]. (c) Lake Challa δD record of leaf wax [11]. (d) Lake Tanganyika δD record of leaf wax [9]. (e) Lake Malawi BSi MAR record [7]. (g) Patate Cave δ18O record, Mauritius [58]. (h) Core GeoB9307-3 from Zambezi Delta [27]. (i) Fe/K ratio of marine core (CD154-17-17K), Eastern Cape [95]. (j) Marine core (CD154-10-06P) elemental ratios of Fe/K, KwaZulu-Natal of South Africa [96]. Yellow shading bars denote prominent cooling interval of YD indicated by Greenland ice core. The site locations of (b–j) are indicated in Figure 1.
Minerals 14 00348 g004

4.2. Humid 8.2 ka in Northwest Madagascar

The drier conditions during the YD in northwest Madagascar are opposite to the observation during the 8.2 ka event. The stalagmite ABC-1 record from a previous study provided precisely dated and high-temporal-resolution reconstructions of the hydroclimate variability covering the period of 8.5–7.5 ka BP [49] to demonstrate that the 8.2 ka event occurred between 8.230 and 8.053 ka BP, characterized by prominent δ18O negative excursions with three calcite layer occurrences inside, indicating prevalent wet conditions (Figure 5g). This overall negative shift in the 8.2 ka event is indeed consistent with other stalagmite records from the same cave [50,62]. The comparison results of the ABC-1 δ18O record (Figure 5g) between the YD and the 8.2 ka event obviously exhibit positive and negative trends, respectively, indicating the opposite hydroclimatic conditions. This phenomenon is unexpected since most records covering these two events normally revealed similar climatic directions because of their similar triggering mechanisms.

4.3. Possible Mechanisms Causing the Opposite Hydrological Conditions between the Younger Dryas and the 8.2 ka Event

The ITCZ migrations, which are in response to the inter-hemisphere temperature difference, have been thought to dominantly drive rainfall variations in East Africa [11,27,33,35,36]. However, given that the ITCZ still would have migrated northwards to the boreal tropics during boreal summer because geological records showed that the reduced boreal summer temperature was insignificant in the Northern Hemisphere during the YD [98,99,100,101] and that a large southward displacement of its mean position is implausible [21,28,92], the drought in tropical East Africa implies that more complex mechanisms rather than a simple southward shift in the ITCZ should have been involved. One possible mechanism is that the opposite hydroclimate conditions could result from a combination of latitudinal shifts of the ITCZ with concomitantly changed convection intensity inside. Stalagmite records from Yemen [102] and the northern Indo-Pacific Warm Pool [12] suggest that the reduced rainfall amount was potentially driven by southward shifts and the weakened intensity of the ITCZ during the YD. If a similar mechanism occurred in tropical East Africa, sites that ITCZ passed over, including Lakes Challa, Malawi, and Tanganyika, preceding the YD could have received less precipitation from the ITCZ during the YD, confirmed by the drought conditions indicated by lacustrine proxy records (Figure 4 and Figure 5). To the east, stalagmite records from Rodrigues Island (Mauritius), which does not currently receive precipitation from the ITCZ, indicated low precipitation through enriched δ18O values during the YD, presumably signifying the influence of a reduced intensity of the ITCZ [58] (Figure 4). Similarly, the reduced rainfall amount in northwest Madagascar during the YD mainly resulted from a reduced intensity within the ITCZ although the ITCZ could pass over biannually.
For southern sites like Lake Chilwa, the Zambezi river mouth, and southwest Madagascar (Figure 1 and Figure 4), where the ITCZ could not pass over even during the YD, the wetter conditions could be linked to the rainfall dipole mode characterized by a negative correlation between subtropical East Africa and tropical East Africa in austral summer, as revealed by current observations [103] and paleoclimate reconstructions [103]. In this dipole, the strength of easterly wind plays an important role by regulating moisture transportation. The simulated result showed that trade wind crossing the equator from the Northern Hemisphere (i.e., easterly wind in the tropics) in the austral summer was strengthened during the YD [21], consistent with the inference that the enhanced ocean-land thermal contrast drove a stronger easterly flow towards the continent [104]. As a result, moisture produced by the southwestern Indian Ocean was suppressed during transportation to north and converged in the subtropics to form a low-pressure trough, thereby causing more precipitation. Consequently, a “dry in the north and wet in the south” mode was expected, as indicated by a range of geological proxies. Indeed, several studies suggested links between El Niño Southern Oscillation and the rainfall dipole in East Africa [103,104]. In addition, the increased rainfall documented by marine cores from the cape of Southeast Africa was potentially associated with the above dipole mode [95,96] (Figure 5).
The same mechanism could likewise have been at play during the 8.2 ka event. The intensity of convection within the ITCZ during the 8.2 ka event should be stronger than that during the YD but weaker relative to the overall early Holocene. As a result, sites located in the northern extent of the ITCZ, like Oman in North Africa, recorded enriched stalagmite oxygen isotope values [3,25] or those with dry conditions documented in ice cores near Kilimanjaro [103], signaling a southward shift and a lower convective intensity of the ITCZ. In contrast, wetter conditions in further-south sites, which are indicated by high-land levels in Lake Chilwa, were dated at ~8.5 ka BP [88] and had depleted isotopic ratios in their northwest Madagascar stalagmite. Since the change in convection intensity within ITCZ could be slight, a southward shift in the ITCZ could lead to a biannual rainy season and hence high precipitation in the study region. On the other hand, a rainfall dipole mode similar to that of the YD could exist in East Africa even though records from southern sites did not show clear excursions due to the limits of resolution or dating (Figure 5). In other word, the boundary between the dry and wet areas has a northward shift because the easterly wind anomaly from the Northern Hemisphere is not as strong as during the YD event. This dipole pattern is in concert with model outputs which showed that the rainfall amount reduced north of 10° S and synchronously increased in northwest Madagascar and further-south sites [16,105,106,107].
Alternatively, the duration redistribution of the ITCZ above its pathway during the abrupt climate events is presumably another driver. The duration of the ITCZ presence over different latitudes in one year is not even and delivers different amounts of rainfall. For instance, the biannual passage of ITCZ currently leads to long and short rainy seasons in tropical East Africa. During the YD, the annual rainfall associated with the ITCZ could be redistributed at different latitudes because of cooler and more prolonged cold seasons in the Northern Hemisphere [108]. This duration is reduced in tropical and northern East Africa because the shorter warm seasons in the Northern Hemisphere make the ITCZ shift northwards and back rapidly. Therefore, a severe drought at sites north of 8–10° S in East Africa was induced [35]. In contrast, the residence time of ITCZ at its south terminus increased in addition to the southward migration, leading to longer rainy seasons and thus more rainfall in regions like the Zambezi River mouth, Lake Chilwa, and southwest Madagascar. Coincidently, a similar north-south anti-phase relation of precipitation was observed in the eastern Indian Ocean regions during the last deglaciation, which was also interpreted as a change in the duration of the rainy season associated with the ITCZ in different regions [12]. For the 8.2 ka event, it is also plausible that the ITCZ rapidly passed northern regions and stayed longer over northwest Madagascar and its south terminus, as above-discussed, causing more precipitation there.
Moreover, sea surface temperatures (SSTs) could also play a competing role in the hydroclimatic variability over East Africa by impacting the supply of atmospheric moisture. Lower SSTs, which tend to reduce the evaporative moisture content of the ITCZ, have been used to interpret the aridity over East Africa during the HS1 [92,97]. Since the YD and HS1 share similarities, the same mechanism could also be at play, and thus, the drought conditions over tropical East Africa are presumably related to the reduced West Indian Ocean SSTs (Figure 5). In addition, recent studies suggest that the Indian Ocean zonal (west-east) sea surface temperature gradient is in close agreement with the hydroclimate proxies in northwest Madagascar and thus drive the millennial-scale climate change from the Last Glacial Maximum [91]. Noticeably, the reconstructed SST gradient over the Indian Ocean exhibited lower excursion during the YD event, in agreement with the drought conditions (Figure 5). As for the 8.2 ka event, the SSTs adjacent to Madagascar were quite high, and thus, sufficient moisture could be supplied (Figure 5), whereas the impact of the SST gradient was obscure because the resolution of SST reconstruction was not enough.

5. Conclusions

Our stable oxygen and carbon records of stalagmite ABC-1 from Anjohibe in northwest Madagascar provide a precisely dated record of rainfall variability mainly covering the period of 13.5–11.3 ka BP. The most prominent feature in the record is the reversed hydrological trends between the YD and the 8.2 ka event. During the YD, enriched δ18O values imply a reduction in precipitation, whereas depleted δ18O values during the 8.2 ka event represent increased precipitation in northwest Madagascar. Possible mechanisms driving these opposite conditions during the YD and the 8.2 ka events were proposed. The weakened intensity of the ITCZ or redistribution of the duration of the ITCZ-related rainy season in one year, accompanied by its southward shift, might contribute to this phenomenon. Furthermore, the Southeast Africa rainfall dipole mode also potentially drove the rainfall pattern when the boundary of dry and wet areas moved. Our record can help refine paleoclimate simulations and provide a better understanding of global circulation and land-atmosphere-ocean interactions in different climate backgrounds.

Author Contributions

Conceptualization, H.L. and P.D.; methodology, P.D. and Y.N.; investigation, H.L., P.D. and H.C.; data curation, H.L., G.K. and P.D.; writing—original draft preparation, P.D. and H.L.; writing—review and editing, H.C., H.Z. and G.Z.; funding acquisition, H.C. and G.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (grant No. 42230812 and 42150710534) and China Postdoctoral Science Foundation (grant No. 2021M692522).

Data Availability Statement

The stable isotope data will be available in the paleoclimate dataset of NOAA at the following website: https://www.ncei.noaa.gov/products/paleoclimatology, accessed on 16 February 2022.

Acknowledgments

We thank the government officer and people in Madagascar for their permission and help in fieldwork. We especially thank the XRD lab of the Frontier Institute of Science and Technology of Xi’an Jiaotong University, China, for accessing to the data and training for X-ray diffraction analysis. We also thank Yassine Ait Brahim and Nick Scroxton for their suggestions on this work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of Anjohibe (red star) and other locations mentioned in the text (yellow solid circles). (1) Lake Challa and Kilimanjaro Plateau. (2) Lake Tanganyika. (3) Lake Mosako. (4) Lake Malawi. (5) Lake Chilwa. (6) GeoB9307-3 marine core, Zambezi River mouth. (7) Core CD154 10-06P, coast of South Africa. (8) Core CD154 17-17K, Eastern Cape coast near the mouth of the Great Kei River. (9) Dunes in Zambia and Zimbabwe. (10) Paleo-shorelines in Central Kalahari. (11) Qunf Cave, Oman. (12) Rodrigues Island, Mauritius. (13) Core GeoB12605-3, Tanzania. (14) Core GeoB12624-1, Tanzania. (15) Tsimanampesotse Cave, Southwest Madagascar. The color of the base map indicates modern average precipitation rates in Africa and surrounding ocean estimated based on the GPCP reanalysis data from DJF (December-January-February) of 1980–2015 (https://www.esrl.noaa.gov/psd/cgi-bin/data/composites/printpage.pl, accessed on 15 February 2022).
Figure 1. Location of Anjohibe (red star) and other locations mentioned in the text (yellow solid circles). (1) Lake Challa and Kilimanjaro Plateau. (2) Lake Tanganyika. (3) Lake Mosako. (4) Lake Malawi. (5) Lake Chilwa. (6) GeoB9307-3 marine core, Zambezi River mouth. (7) Core CD154 10-06P, coast of South Africa. (8) Core CD154 17-17K, Eastern Cape coast near the mouth of the Great Kei River. (9) Dunes in Zambia and Zimbabwe. (10) Paleo-shorelines in Central Kalahari. (11) Qunf Cave, Oman. (12) Rodrigues Island, Mauritius. (13) Core GeoB12605-3, Tanzania. (14) Core GeoB12624-1, Tanzania. (15) Tsimanampesotse Cave, Southwest Madagascar. The color of the base map indicates modern average precipitation rates in Africa and surrounding ocean estimated based on the GPCP reanalysis data from DJF (December-January-February) of 1980–2015 (https://www.esrl.noaa.gov/psd/cgi-bin/data/composites/printpage.pl, accessed on 15 February 2022).
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Figure 2. Age model constructed by MOD-AGE [67] using U-Th ages (left) and image of stalagmite ABC-1 (right). Median growth (black line) and the 2.5% and 97.5% confidence envelopes (gray shading area) based on Monte Carlo simulations are shown together with the individual 230Th ages (blue dots) and 2σ errors (blue vertical bars). Details of the 230Th age data are given in Table 1. The blue dash lines indicate the location of hiatuses.
Figure 2. Age model constructed by MOD-AGE [67] using U-Th ages (left) and image of stalagmite ABC-1 (right). Median growth (black line) and the 2.5% and 97.5% confidence envelopes (gray shading area) based on Monte Carlo simulations are shown together with the individual 230Th ages (blue dots) and 2σ errors (blue vertical bars). Details of the 230Th age data are given in Table 1. The blue dash lines indicate the location of hiatuses.
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Figure 3. Mineral identification, trace element, and stable isotope results. The left subpanel shows depth vs. from top to bottom: variations in mineralogy indicated by different color triangles according to X-ray diffraction estimations (a), Sr/Ca (grey) and Mg/Ca (pink) ratios (b), inferred mineralogy based on X-ray diffraction estimations and trace element result (c), raw carbon isotope records (grey, d) corrected to calcite values using 1.7‰ offset (olive, d), and raw oxygen isotope records (grey, e) corrected to calcite values using 0.8‰ offset (blue, e). The right subpanel shows chronology vs. variations in mineralogy (f), δ13C (g), and δ18O (h). The black dots and associated error bars indicate the 230Th dating result ages and 2σ error.
Figure 3. Mineral identification, trace element, and stable isotope results. The left subpanel shows depth vs. from top to bottom: variations in mineralogy indicated by different color triangles according to X-ray diffraction estimations (a), Sr/Ca (grey) and Mg/Ca (pink) ratios (b), inferred mineralogy based on X-ray diffraction estimations and trace element result (c), raw carbon isotope records (grey, d) corrected to calcite values using 1.7‰ offset (olive, d), and raw oxygen isotope records (grey, e) corrected to calcite values using 0.8‰ offset (blue, e). The right subpanel shows chronology vs. variations in mineralogy (f), δ13C (g), and δ18O (h). The black dots and associated error bars indicate the 230Th dating result ages and 2σ error.
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Figure 5. Comparison of the ABC-1 δ18O record (g) with different indicators in other studies. (a) NGRIP ice core [1]. (b) Lake Challa δD record of leaf wax [11]. (c) δD record of leaf wax [9]. (d) Log (Ti/Ca) record from Core GeoB12605-3, off Tanzania [36]. (e) Log (Ti/Ca) record of Core GeoB12624-1 from mouth of the Rufiji River [35]. (f) δ13Calkane record of Lake Malawi [33]. (h) Stalagmite ANJB-2 δ18O record from Anjohibe, Madagascar [55]. (i) Tsimanampesotse Cave δ18O record, southwest Madagascar [37]. (j) Patate Cave δ18O record, Mauritius [58]. (k) Hydrogen isotope compositions of the n-C31 alkane in GeoB9307-3, off the Zambezi River mouth [27]. (l) UK′37 derived SSTs from the marine sediment core in the Mozambique Channel [97]. (m) Tropical western Indian Ocean SSTs using the Last Glacial Maximum reanalysis [91]. (n) Marine core (CD154-10-06P) elemental ratios of Fe/K, KwaZulu-Natal [96]. (o) Fe/K ratio of marine core (CD154-17-17K), Eastern Cape [95]. Yellow shading bars highlight the central YD and 8.2 ka event. Locations of East Africa records are shown in Figure 1. Hydrological conditions are indicated by arrows with notation.
Figure 5. Comparison of the ABC-1 δ18O record (g) with different indicators in other studies. (a) NGRIP ice core [1]. (b) Lake Challa δD record of leaf wax [11]. (c) δD record of leaf wax [9]. (d) Log (Ti/Ca) record from Core GeoB12605-3, off Tanzania [36]. (e) Log (Ti/Ca) record of Core GeoB12624-1 from mouth of the Rufiji River [35]. (f) δ13Calkane record of Lake Malawi [33]. (h) Stalagmite ANJB-2 δ18O record from Anjohibe, Madagascar [55]. (i) Tsimanampesotse Cave δ18O record, southwest Madagascar [37]. (j) Patate Cave δ18O record, Mauritius [58]. (k) Hydrogen isotope compositions of the n-C31 alkane in GeoB9307-3, off the Zambezi River mouth [27]. (l) UK′37 derived SSTs from the marine sediment core in the Mozambique Channel [97]. (m) Tropical western Indian Ocean SSTs using the Last Glacial Maximum reanalysis [91]. (n) Marine core (CD154-10-06P) elemental ratios of Fe/K, KwaZulu-Natal [96]. (o) Fe/K ratio of marine core (CD154-17-17K), Eastern Cape [95]. Yellow shading bars highlight the central YD and 8.2 ka event. Locations of East Africa records are shown in Figure 1. Hydrological conditions are indicated by arrows with notation.
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Table 1. 230Th dating results. The error is 2σ.
Table 1. 230Th dating results. The error is 2σ.
No238U
(ppb)		232Th
(ppt)		230Th/232Th
(Atomic × 10−6)		δ234U
(Measured)		230Th Age (yr BP)
(Corrected)		Depth
(mm)
AB1-13596±4.15026±1011185±245.8±1.211,368±38264
AB1-26825±33.97967±1641429±303.2±2.211,489±74304
AB1-36576±33.911,558±239958±201.3±2.211,620±82322
AB1-42696±2.62433±491880±382.7±1.511,717±36343
AB1-53879±4.51124±235894±1203.6±1.511,809±29366
AB1-66573±9.6924±1912,113±2502.9±1.411,783±29368
AB1-7188±0.2930±19358±84.5±1.212,098±135371
AB1-8350±0.5885±18705±144.5±1.512,294±72374
AB1-9113±0.1385±8544±126.2±1.612,711±155380
AB1-10144±0.3237±51132±244.1±2.512,919±88426
AB1-11187±0.2372±7947±193.1±1.313,039±56442
AB1-12206±0.2626±13625±134.0±1.313,115±73473
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Duan, P.; Li, H.; Kathayat, G.; Zhang, H.; Ning, Y.; Zhu, G.; Cheng, H. Opposite Hydrological Conditions between the Younger Dryas and the 8.2 ka Event Revealed by Stalagmite from Northwest Madagascar in East Africa. Minerals 2024, 14, 348. https://doi.org/10.3390/min14040348

AMA Style

Duan P, Li H, Kathayat G, Zhang H, Ning Y, Zhu G, Cheng H. Opposite Hydrological Conditions between the Younger Dryas and the 8.2 ka Event Revealed by Stalagmite from Northwest Madagascar in East Africa. Minerals. 2024; 14(4):348. https://doi.org/10.3390/min14040348

Chicago/Turabian Style

Duan, Pengzhen, Hanying Li, Gayatri Kathayat, Haiwei Zhang, Youfeng Ning, Guangyou Zhu, and Hai Cheng. 2024. "Opposite Hydrological Conditions between the Younger Dryas and the 8.2 ka Event Revealed by Stalagmite from Northwest Madagascar in East Africa" Minerals 14, no. 4: 348. https://doi.org/10.3390/min14040348

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