How Young is a 300-Year-Old Wetland? The Case of the Pantanal Marimbus, Chapada Diamantina, Brazil

The Chapada Diamantina, in Northeastern Brazil, is one of the few places where one can nd drylands with a backswamp containing hundreds of dead deciduous trees in the oodplain. During the 18th century, the region was globally important due to the exploration of mineral resources. The death of these trees was caused by mining activities that silted the main river, leading to the impoundment of the tributary river, and resulting in a wetland known as Pantanal Marimbus, having as indicators: (i) backswamp morphological feature that remains permanently ooded in the axis of the uvial course, and (ii) alluvial fans concentrated in one footslope area where mining activities at the Chapada Diamantina were also concentrated. The hydrological and sedimentological behavior was investigated to multi-methods. By analysing four different samples from the bark and core of the same tree, we obtained calibrated radiocarbon dates within the 18th century. For no robust dendrochronology could be performed, a simple sequence model was built, revealing a high probability that the tree lived until approximately 1700 AD. 14 C-AMS measured pioneering possible to evaluate the 300-years-old wetlands juvenile evolutionary state. to verify the level differences in the from Shutther The present used processed SRTM models with 30-meter resolution. The water base level of the hydrological data (Fertém Station) was compared to the SRTM, and satellite images were interpreted to then determine the maximum ooding area in the Marimbus wetland. Multitemporal satellite images acquired 2021, from the Global Surface Water Explorer data platform provided accurate, up-to-date, high-resolution


Introduction
Wetlands are transitional lands between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water 9 . Globally, many wetlands form in broad valleys on low slopes, some being permanently ooded, while others are only ooded seasonally 51 . The rst wetlands date back to the Silurian Period, being environments where many ora and fauna species had to adapt and therefore being importante for the evolution of life on Earth 16 . Wetlands continue to decline globally, both in area and in quality, and are sensitive to environmental and climatic changes over decadal to millennial timescales 21 . Wetlands contribute to water puri cation, ood control, and food production, both land-and water-based, and the maintenance of high levels of biodiversity and ecosystemic relevance to society 51 .
From a geomorphological point of view, wetlands represent a complex plain of alluvial-detrital coalescence 1 . River ood plains develop through erosion processes and through deposition of sediment on adjacent lands during oods 37 . Geomorphological studies in wetlands include analyses of landform shapes, i,e., quanti cation of surface and near-surface processes (e.g. runoff, sedimentation, surface water changes) that shape landforms, and landscape changes that occur in response to factors such as climate change and human activities 54 .
Investigations may be directed towards reconstructing past processes, towards understanding present-day processes, or towards anticipating future processes 11 . Human activities such as mining, agriculture or settlement occur in or around wetlands and can promote increased erosion and sedimentation. Radiocarbon dating has been widely used to determine sedimentary dynamics and infer the origin and monitoring of wetlands, oodplains and lakes 31  The Chapada Diamantina mountain range, the region with the highest elevations in northeastern Brazil, is an example of a place where we nd a backswamp in drylands that remains permanently-to-seasonally ooded. The Pantanal Marimbus (Marimbus wetland), as it is known, contains hundreds of dead deciduous trees in what should be the riparian zone. Signs of avulsion show that there has been a relatively sudden change in channel belts and the construction of alluvial fans. It is known that the region has been subjected to intense diamond exploration by independent miners (colloquial Portuguese: garimpeiros) with report that such activity generated a profound enviromental and economic change in the Chapada Diamantina.
The discovery of diamonds in Chapada Diamantina (State of Bahia) is controversial. The earliest records of diamonds were made by travelers Spix and Martius, in 1820. O cial records began in 1844, a rather ephemeral cycle due to the discovery of large deposits in Kimberley, South Africa, in 1867 3 , which forced the fall in mineral prices in the market and caused Brazil to lose its hegemony of world's largest diamond producer, at the height of the mining cycle. However, historical data indicate that the clandestine search and exploitation of the precious stone predated the 19th century 33 . It is possible that the existence of diamonds in the Chapada Diamantina has been known since the 18th century, based on the ban on exploitation by Brazilian viceroy, dated 1732 17 . Diamond mining in this region has been affected by long periods of drought causing a drop in production. Later, with the expansion of industrial activity in Europe and the United States, the excavation of the London Underground and the opening of the Panama Canal, carbonado (a variety of diamonds) brought new life to the local economy. The carbonado is a characteristic diamond of the Chapada Diamantina, dark and dull in color, of high hardness, with high resistance to friction and heat. This spurred a new diamond exploration cycle in the region that lasted until the rst decade of the 20th century 3 , when its production declined again.
In general, diamond extraction was carried out in a rudimentary manner, known as sawmill mining (open pit and subsurface), where the diamond gravel derived from the conglomerates was excavated on the anks of the mountains. This action led to the rapid transfer of sediments from the Sincorá range, causing siltation and hydrodynamic change in the rivers.
There are no historical reports of natural events that could be responsible for the intense silting up of rivers. On the contrary, the people currently living in the region comment that their ancestors originally associated the Marimbus wetland origin with diamond mining. The Theodoro Sampaio's expedition (1879-1880) already mentioned in rich detail the presence of a "Lagoa Grande" (Great pond) referring to a large ooded area, with location and extent that are similar to the current Marimbus wetland features 42 . However, the date of the Marimbus wetland origin has always been uncertain.
The main objectives of this work are: (i) to verify if there are geomorphic features that indicate the change of uvial environment to a wetland, (ii) to determine hydrogeomorphological behavior, (iii) to use isotopic data to assess the origin of water, whether surface or groundwater, and (iv) to determine by radiocarbon dating ( 14 C-AMS) the date and causes of the dead trees in the Marimbus wetland.

Physiographic Settings
The   (Fig. 1a). The period of highest rainfall occurs between November and April. When torrential rains occur on the headwaters of the mountain, the water ows quickly through the canyons, carrying a large volume of sediment. However, prolonged periods of drought can occur and cause largescale forest res. The great catastrophic drought of 1877, for example, forced major migrations from the Chapada Diamantina, taking part in the decline of diamond mining 15 .
With regard to vegetation, the rocky elds cover large areas in Chapada Diamantina, usually above 1000 m. They are characterized by their predominantly shrubby size, which grows on rocky outcrops or shallow soils. The oristic composition in these environments is strongly conditioned by the substrate, besides the altitude and the precipitation. Arboreal forests occur in fractures.

Hydrogeomorphological behavior
The Fertém station data show Marimbus wetland hydrogeomorphological regime and maximum potential ooding ( Fig. 3a), namely: the uvial discharge; the variability of water levels (seasonally and yearly extent); how often it oods and dries (frequency); how far the water spreads (extent); The catchment area of the Marimbus wetland covers 10.111 km 2 , strongly in uenced by the mountainous relief, which reduces the permanence of water in this sector of the basin and intensi es the ow. The average river discharge in the Marimbus wetland is relatively low, at 29.5 m 3 s -1 and sedimentation is proportional, <5 ton.day -1 .
However, discharge peaks of 878 m 3 s -1 are observed (Fig. 2a), which move a higher sediments load than during long periods under normal conditions, reaching 962 ton.day -1 . These peaks are much lower than those found for average Brazilian and worldwide rivers, if proportionally considered the ratio between catchment areas versus solid and liquid river discharge 24 .
The Marimbus wetland has maintained an average water level of 1.20 m (Fig. 2b). The oodplain is quite uneven with many shoals and low margins. This means that a rise of only 2 m in water level is enough to ensure the maximum lateral ooding of the wetland, which occurs 25% of the time, with several ooding cycles exceeding 5 m in height. This frequent waterlogging led to the death of the trees and new composition of vegetation formations. Most wetlands are shallow depressions and small catchments areas 8-9 .
The highest elevation period of the Marimbus wetland water level extends from November to April, while the other months are considered as dry period, whose ood frequency rarely evades this pattern of seasonality (Fig. 2c).
Several droughts reduced their levels drastically (December 2011-to-December 2013), but the freshwater remains. The opposite moisture event occurred between October 1977 and May 1979, when this wetland was ooded during all months, above the long-term monthly average. Both events are considered not seasonal. During the dry season, a mosaic of sandy islands emerges (Fig. 3a). During drought events, the alluvial fans are exposed and only isolated sedimentation rings remain ooded (with rounded dolines resemblance). The latter contain trapped silt and clay facies, while along the active channels there is ne to coarse sand. The overbank deposition enabled areas of shallow water to form in the adjacent backswamps. The abandoned meanders indicate these hydrodynamic energy oscillations in the ooded area  , creating new channels on the oodplain and causing the abandonment of the old channel (avulsion).
The delimitation of the Marimbus wetland area was complex. We used a colored composition containing the band 4 (0.77-0.89 µm) commonly used for delineation of water bodies, and compared with SRTM contour lines extracted with 1 m equidistance from the 320 m above sea level (base station). Both boundaries extracted from the images show similarity in the 6 m maximum hydrological ood level (326 m asl), which occurred only in 1960 (see Fig. 2a) and its corresponding ood area of ≈58 km 2 .
The Marimbus surface water dynamics has been recorded from coarse-spatial-resolution satellite observations, and higher-resolution seasonality maps using all Landsat images over multiple decades have been used to map seasonality and surface water changes. Hydrogeomorphological behavior analysis showed that Marimbus is a freshwater wetland, ooding periodically, currently located in a lithological depression. The three sectors (northern, central and southern) show changes over time and space, with the southern portion and the Encantada pond being the areas with the most permanent water levels. All areas were subject to ephemeral and seasonal oods, between 1984 and 2021 (Fig. 3b). This land supports predominantly hydrophytes, the substrate is mostly undrained hydric soil, and is saturated with water or covered by shallow water at some time during the growing season of each year.  Generally, this enrichment can be the result of the recharging of cloud rains that are not generated in the region, with several consecutive episodes. This causes a rapid depletion of the 18 O in the cloud generating positive excess deuterium. The data collected in the southern portion, points MB8 to MB18, showed slightly positive deuterium values, closer to the value of the GML coe cient, indicating that the meteoric waters were their main source of recharge. The MB16 and MB17 points, collected in the southern Marimbus wetland, show the lowest values of deuterium for this data group: +7.9 and +7.6‰, respectively. The MB7 point, collected at the con uence of the São José and Santo Antônio rivers, showed the lowest value of positive deuterium due to its proximity to the sedimentation rings of the central portion of the wetland. It suggests the interaction between groundwater and river, which maintains the minimum levels in the dry period and ampli es the quotas in the rainy periods.

The water's source
The similarity in isotopic information at these points is due to the marked in uence of precipitation on runoff.
These waters are typical of the oodplain which remains saturated for extended lengths of time (backswamps) and is often isolated from the river channel as a result of aggradation occurring elsewhere on the oodplain 25 .
The waters in the karstic aquifer that feed into the springs of the Santo Antônio river in the Irecê carbonate basin range from calcic sodium chlorinated composition to calcium bicarbonate 40 . The isotopic composition of aquifer waters refers to the natural waters of an isolated karstic system, its isotopic signature distinct from the waters of the Marimbus wetland (Fig. 4). They are rounded shaped ponds typically found in wetlands and oodplains too, but theses body waters are alkaline salines and present pH values of up to 10, with the presence of bicarbonate, chlorinated and sodic waters 5 . Some of these ponds are isolated from pluvial surface ow and are characterized by white-sand beaches and brackish to saline water during Holocene-Pleistocene.

Radiocarbon dating geomorphic change
The radiocarbon results for each tree ring sample are presented in table 1. The piece of trunk used for radiocarbon ( 14 C-AMS) analyses was sampled along the growth rings sequence, so that an age model could be built. This was done because the independent dating of the bark would result in a wide probability range, covering the industrial period when the input of fossil carbon has diluted the atmospheric radiocarbon concentration, preventing precise dates to be estimated 47 .
By dating four different samples from the bark and core, it was possible to obtain radiocarbon dates that correspond to the 18th century. For no robust dendrochronology could be performed, a simple sequence model was built, revealing a large probability that the tree lived until approximately 1700 AD (Fig. 5). Based on the dating results there would still be a slight probability of the bark reaching 1800 AD. However, the size of the sample indicates that, even if some rings were missing, it could not represent more than 100 years of growth for the Hymenolobium sp. Previous work on the growth rates of some Amazon trees revealed that such species usually grows less than 40 cm y -1 32 . Therefore, the death of the tree most probably took place around 1700 AD.
Recents radiocarbon analysis showed that landslides were responsible for creating two lakes in the western United States after earthquake events, including the A.D. 1700 Cascadia earthquake. Generally, mountainous settings commonly trigger thousands of landslides, and slope failures are typically signi cant for landslidedammed aquatic enviroments 46 . As there was never any record of earthquakes in Chapada Diamantina, we strong suggest that Marimbus wetland were formed approximately 1700 AD, by mining activities that silted the main river, leading to the impoundment of the tributary river.
It has been frequently stated that the world has lost 50% of its wetlands (or 50% since 1900 AD). The reported long-term loss of natural wetlands averages between 54-57% but loss may have been as high as 87%, since 1700 AD 11   Satellite images show that alluvial fans on the Sincorá range footslope are concentrated between parallels 12º20'00"S and 13º00'00"S (Fig. 6). Historical data showed that diamond mining activities at Chapada Diamantina also concentrated approximately between these same parallels 41 . Outside this area there was not diamond exploration at Chapada Diamantina, and the rivers ow from the mountain without accumulating sediment.
The alluvial fans divide the Marimbus wetland into three sectors, all located at the con uences of the rivers. A narrow and elongated alluvial fan separates the northern portion from the central portion at the con uence of the São José and Santo Antônio rivers. Another alluvial fan with elongated radial axis extending for approximately 1.5 km separates the central portion from the southern portion, at the con uence of the Garapa and Santo Antônio rivers (Fig. 7). This fan system is broadly divisible into three parts: (i) an upper entry corridor, approximately 6 km long and 40 m wide; (ii) a central zone of seasonal swamps transected by several distributary channels con ned by densely vegetated banks, and (iii) a lower zone of perennial swamps where ow is mostly uncon ned. In the nal stretch, the Santo Antônio river (tributary) was dammed by the Paraguaçu alluvial fan (main river) because, due to its intense siltation, it became topographically higher than the tributary river (Fig. 8). We suggest that the transfer of sediments from the mountains to the con uence of rivers was responsible for allowing the river system to become a wetland.
If we use radiocarbon dating of the geomorphic change of the Marimbus wetland as a marker, it is observed that the alluvial fans present exceptionally high sedimentation rates and could not have occurred in natural conditions. In this case, 300 years would be a short time to deposit this volume of sediment, where the sedimentation rate should be 6 to 8.6 cm y −1 . Sedimentation rates > 1 cm y −1 are associated with great rivers (Walling and Fang 2003). Soundings indicated alluvial fans maximum thickness, between 18 to 26 m 41 .
The combination of intense river discharge, high slope and the mining activity may explain the alluvial fans ( Fig. 8), forming placers deposits that are still active today, and where coridon, rutile, cianite, limonite and turmaline can be found in the diamonds levels. The occurrence of gold is not very signi cant. The volume of these alluvial fans containing only diamond pebble-supported has been estimated at 20 x 10 6 m 3 , where: Paraguaçu alluvial fan has been 6.8 x 10 6 m 3 (Fig. 8), Santo Antônio alluvial fan has been 10 x 10 6 m 3 and São José alluvial fan has been 3.2 x 10 6 m 3 41 .
The geomorphic change dating (this work) suggests that the exploration of mineral resources has existed since the 18th century in the Sincorá mountain range. In this scenario, it is not clear whether the existence of diamonds in the Chapada Diamantina has been known since the 18th century. It is possible that alluvial fans originated from the gold search phase and consequently formed the Marimbus wetland. Only after 1844 did Chapada Diamantina intensify exploration and attract thousands of miners to the region, where Chapada had its name linked to Diamonds (Toponymy: Chapada roughly means sedimentary plateau on Precambrian rocks, and Diamantina concerning Diamonds). This date only symbolizes one fact, due to the fact that the discovery of the mineral was kept secret and occasionally a diamond buyer was forced to reveal its existence 49 .
Historical documents show that between the 17th and 18th centuries, explorers circulated intensively in the coastal and semi-arid region of the State of Bahia (site of the discovery of Brazil, in 1500 AD) in search of metals and precious stones. In reality, this movement dates back to the 16th century. The rst expedition entered the Paraguaçu river, in 1559 35 and reached Chapada Diamantina region in the 17th century. The search for mineral wealth was hard and slow and lasted a long time without major discoveries.
Early indications of Brazil's mineral potential were sporadic, but there is evidence that crystals were found in Bahia within a century of Columbus's discovery of the New World. In one of the earliest descriptions, historian Pero de Magalhães Gândavo (1576) mentioned the existence of "certain mines of white stones such as diamonds". In another account, Gabriel Soares de Sousa (1587) noted that ne, eight-sided crystals (possibly diamond) had been found during the dry winter months along certain rivers 48 .
The search for diamonds, gold and other valuable natural products was a major driving force for the exploration and colonization of the interior of the country in the 17th and 18th centuries 49 . The discovery of gold in the state of Bahia was the result of much investment by the Portuguese government which, throughout the 17th century, stimulated, subsidized and gave rewards to anyone who ventured in search of metals and precious stones 14 .
The largest results of the expeditions were achieved rst with gold, found in the 18th century, then diamond, whose rst deposits were discovered in the early 19th century. Portugal moved aggressively to control the area, restricting gold and diamond mining and imposing high taxes.
Despite efforts by the crown, clandestine mining and diamond smuggling increased 49 .

Hydrogeomorphology
The hydrogeomorphological behavior was calculated by last gauging station in the Santo Antônio river (Fertém

Radiocarbon 14 C-AMS
Radiocarbon Accelerator Mass Spectrometry ( 14 C-AMS) was based on a sample from the dead forest (obtained February 2014), in the southern portion of the swamp (Lat 12°45'16.57"S / Long 41°18'7.48"W). The sampled tree was characterized as being from the genus Hymenolobium sp. Sample preparation and measurement were performed at the Radiocarbon Laboratory of the Federal Fluminense University, Brazil (LAC-UFF). A sample of approximately 2 cm in width was analyzed with a microscope and four subsamples were collected from three growth rings of the trunk plus the bark. Each of these samples was prepared separately so that a sequence model could be constructed.
Samples were washed in ultra-pure (UP) water and subjected to a chemical treatment for the extraction of cellulose. The Acid-Base-Acid (ABA) protocol comprised a sequence of treatments with 1.0M HCl acid for 2 h, 1.0M NaOH base for 1h (repeated until the supernatant was clear) and a nal treatment with 1.0M HCl acid for 2h (all treatments at 90°C). Holocellulose was then extracted with 1.0M NaClO 2 and 1.0M HCl at 70°C for about 4h and repeated until the sample was clear 44 . Samples were rinsed with UP water and dried. Next, they were combusted in independently sealed quartz tubes with CuO and Ag wire at 900°C for 3h. After puri cation of the obtained CO 2 , samples were transferred to pyrex tubes with Zn,TiH 2 and Fe, torch sealed and heated at 550°C for 7h in a mu e oven 55 Declarations