Ecologists have proposed that montane grassland-shola (stunted evergreen forest) mosaics in the Western Ghats may represent alternative stable vegetation states. But paleoecology investigations seldom consider this framework, especially the role of short-term disturbances (fire, intense drought) other than long-term climatic changes, that can cause vegetation switches in landscapes with alternative vegetation states. The Sandynallah valley that hosts one of the oldest peat accumulations in the world at >50 kyr has been central to the reconstruction of paleovegetation in the montane Nilgiris, Western Ghats. Although the peat-forming vegetation here (dominated by sedges) is a unique vegetation state, its contribution to the paleovegetation signal has not been explicitly considered. We propose a conceptual framework of a tri-stability landscape with sedgeland on the valley floor, grassland on the hill slopes and shola vegetation in the boundary between sedgeland and grassland. While frost prevents shola saplings from establishing in grassland, waterlogging provides a barrier for their establishment in sedgeland, thus maintaining these distinct vegetation states under the same climate. We investigated the stable carbon isotope signatures of the cellulose fraction from two well-dated peat cores (Cores 1 and 2) collected from ~170m apart in the Sandynallah valley within the alternative stable states framework. We find that Core 1, which is closer to the boundary of valley and hill slope, shows dynamic switches between sedgeland and shola whereas Core 2, located in the centre of the valley floor, represents a stable sedgeland state. The vegetation switches and maintenance mechanisms at Core 1 is connected to a disturbance (fire) and to changing climate while Core 2 seems to be responding primarily to climatic changes. The simultaneously distinctive vegetation states in Cores 1 and 2 at such close proximity within the same valley is the first record of alternative stables states in the past in the Western Ghats.

Banded Iron Formations (BIFs) are archives of Precambrian seawater composition. Presence or absence of negative Ce anomaly (Ce/Ce*) in BIFs has been widely used to understand paleo-redox conditions on the Earth’s surface in the Precambrian. However, whether the extremely negative Ce anomaly associated with the BIFs reflects a primarily depositional signature or not has been questioned and it has been suggested that such signatures could also arise from secondary alterations.1 We report elemental and Nd isotopic data for BIFs and associated clastic rocks from the Sirsi region in southern India. Major and trace element compositions of these BIFs were measured using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS, X series II) while Nd isotope ratio (143Nd/144Nd) measurements were performed using a Thermal Ionization Mass Spectrometer (TIMS, Triton Plus), both at the Centre for Earth Sciences (CEaS), Indian Institute of Science Bangalore, India. The BIF samples are sub-divided into two groups based on their REE+Y (REY) compositions. The group-1 BIFs show seawater-like REY pattern with HREE enrichment over LREEs and super-chondritic Y/Ho (41-52). These BIF samples also lack significant negative Ce anomalies. In contrast, group-2 BIFs show high LREE/HREE enrichment, negative Ce anomaly, and sub-chondritic Y/Ho. Very high values of La/Yb in the group-2 BIFs cannot be explained by simple two-component mixing of basement rock (Dharwar TTG) and pristine Sirsi BIFs. Instead, fluid-rock alteration by LREE enriched, and Ce depleted fluid could explain the observed REY variations. We further utilized Sm-Nd isotope systematics to calculate the timing of this alteration event. These BIFs show lowest RSD (%) in their initial 143Nd/144Nd composition around 0.6 Ga, which we consider as the time of alteration event which affected the Sm/Nd of these rocks. The timing of alteration event coincides with the Pan-African orogeny which had regionally affected the Greater Dharwar Craton. The associated red shales are also characterized by high LREE/HREE ratios and negative Ce anomalies. These shales also show very high Chemical Index of Alteration (CIA) values (83-99) suggesting high degree of chemical weathering. [1] Bonnand et al, (2020) Earth and Planetary Science Letters

The Sandynallah valley (Western Ghats, India) features one of the oldest peat accumulations in the world at >50 kyr and has been central to the reconstruction of late Quaternary paleoclimate using paleovegetation changes in the forest-grassland vegetation mosaic that coexist here. It is well-known that short-term disturbances (fire, frost, intense drought) can also cause vegetation switches when multiple stable states exist, but this framework has seldom been considered in paleoecology investigations. Using stable carbon isotope signatures (relative C3-C4 vegetation abundance) on the cellulose fraction from two well-dated peat cores ~170 m apart - Core 1 closer to the hillslope (32000 years old) and Core 2 from the centre of the valley floor (45,000 years old) - we looked at paleovegetation changes and the implications for paleoclimate reconstruction within the alternative stable states framework. Charcoal data from another sediment profile from the same valley was used to correlate with paleofires. We propose that the valley floor is bistable, switching between peat-forming vegetation ‘sedgeland’ and montane stunted evergreen forest ‘shola’, maintained by level of waterlogging. Core 1 shows shola-sedgeland dynamics with vegetation switching at c.22ka from shola (possibly due to fires) to a prolonged unstable state until 13 ka sustained by low waterlogging. Following a hiatus c.13-7 ka, sedgeland dominates, with a shift into shola at 3.75 ka driven by increasing aridity. Core 2 shows a relatively stable signature, enriched in C3-vegetation in the last glacial (45-20 ka) compared to the Holocene. Given temperature is the primary driver of abundance in C3-C4 mixed-grasslands, C4 dominance beginning c.18.5 ka followed by C4 enrichment is indicative of deglacial warming that continues into the Holocene except for a departure at ~10 ka. The record at Core 2 is indicative of changing climate while Core 1 shows disturbance-based vegetation dynamics. The simultaneously distinctive vegetation states in Cores 1 and 2 within the same valley is the first record of alternative stable states in the past in the montane tropics. Our results point to the need to account for short-term disturbances and site attributes before ascribing vegetation changes to changing climate in alternative stable states landscapes.