Potrok Aike


Potrok Aike is a maar in the Patagonian province of Santa Cruz, Argentina, which contains a brackish lake. It has a roughly square shape and is about wide. The lake is fed by groundwater and occasional inflows through dry valleys, and its water levels have fluctuated over the course of its 770,000-year-long history between overflow and near desiccation. Recent climatic variability has left a series of terraces and shorelines, both around the lake and submerged under its waters. The variability is driven mainly by changes in wind speed, which change the evaporation rates. Usually, cold periods correlate to higher and warm periods to lower water levels in Potrok Aike.
The lake is embedded in a dry steppe region and is one of the few open water bodies there. It has been studied as a site for long drill cores and palaeoclimate records of the PASADO/SALSA project, which in 2002 and 2003 took about half a kilometre of drill cores from the lake. These drill cores were used to reconstruct the climate and environment of the region, including volcanic activity and changes in Earth's magnetic field.
The region around Potrok Aike was periodically glaciated before the maar formed, but later glaciations did no longer reach it. Sparse volcanic activity built the Pali-Aike volcanic field, which was active until 10,000 years ago east of Potrok Aike. About 12,500 years ago humans first arrived in Patagonia. They used the resources around Potrok Aike, including rockshelters, to dismember animal kills. The lake itself is inhabited by algae and underwater plants.

Etymology and human geography

The name is part-Tehuelche and part-Spanish: Spanish potro means, and Tehuelche aike means a place where people stop and store resources. This may indicate that Potrok Aike was a stopping point for local people, as it has a reliable water supply. The region was inhabited by Tehuelche people before Westerners arrived at the end of the 19th century. Sheep grazing is widespread in the region.
Politically, Potrok Aike is in Argentina's Santa Cruz Province. The lake can be reached from the National Route 40 between Río Gallegos and Río Turbio, which passes about north of Potrok Aike. The border to Chile is about south of Potrok Aike. The "Diego Ritchie" police station lies southwest of the lake, a weather station about northwest and there is a field station of Argentina's National Agricultural Technology Institute.

Geography and hydrology

Potrok Aike is a wide, roughly square-shaped lake in southern Patagonia, north of the Strait of Magellan and west of Río Gallegos. There are few permanent lakes in this region; another one is Laguna Azul.
The lake has a shape resembling a simple pot, typical for volcanic lakes, and covers a surface area of at about above sea level. The lake floor has a bowl shape, with a shallow platform at depth, a steep drop to depth and a flat lake floor at depth. Water levels fluctuate up to between seasons; interannual variability reaches. The lake is a wide and deep volcanic crater embedded within flat terrain.
Beaches are formed by pebbles, sand and silt; there are gravel ridges formed by longshore transport. Water currents eroded the eastern coast. Potrok Aike is surrounded by flights of lacustrine terraces, both above and below the present-day water level; the highest paleoshoreline is above the present-day lake surface. Alluvial fans developed on the terraces, while wind moves sediments to form dunes. Submerged shorelines including submerged beach ridges are found at depth, and there are erosional unconformities formed by past lowstands. A now-submerged river delta may occur in the southeastern part of the lake. The "Policía" scoria cone and "Bandurrias" lava flow crop out on the southwestern corner of the lake and have been eroded by waves. At higher water levels, large embayments would form in the northwestern and southwestern parts of the lake, and a small bay on the eastern side.

Hydrology and watershed geomorphology

The waters of the lake are well-mixed by wind, which can form waves up to high and produces a water circulation in Potrok Aike that erodes lakefloor sediments. Water temperatures range between with little differences by depth; temperatures exceeding are rare. The lake contains about of brackish water, with a high dissolved oxygen content throughout the water column and a medium to high concentration of nutrients like phosphorus. The Secchi depth is about. Being a closed lake, water levels fluctuate with climate conditions and also with the conditions of the surrounding landscape ; rising/falling temperature, stronger or more westerly winds/weaker or more easterly winds and lower/higher precipitation decrease/increase water levels, while cloud cover plays a subordinate role.
The watershed area covers more than and reaches into Chilean territory; the ephemeral Bandurrias Creek enters Potrok Aike from the west through an U-shaped cascade. Especially on the eastern side there are arroyos, some of which cut lake terraces and which carry water mainly during snowmelt. There is significant groundwater inflow which buffers water levels although its exact significance to water levels is unclear. About half of its inflow seeps out through groundwater.
Most of the terrain is formed by moraines and outwash fans of the "Patagonian Gravel" Formation, with outcrops of the underlying Santa Cruz Formation in the southern lake terraces. Lava plateaus, maars and scoria cones are widespread. Numerous ephemeral lakes and dunes formed by lake sediments dot the landscape around Potrok Aike; unlike them, the great depth of Potrok Aike keeps it water-filled year round. Former meltwater channels from ancient glaciations, one of which passes just west of Potrok Aike and is occupied by Robles Creek and Bandurrias Creek, cross the region. The area is arid, has not been glaciated for 800,000 years and volcanic activity has been minor, leaving wind and cryogenic processes as the only landscape forming agents; it is thus well-conserved.

Sediments

The bottom of the lake is covered by up to thick sediments. They are mostly carbonates, along with wind-blown and suspended material. In cores, they appear as ball and pillow structures, laminated silts, laminated silts with embedded silt and sand layers, lightly coloured silt layers, normally graded beds, and sand with fine gravel layers and no other structure. Concretions formed perhaps by microbial activity, mollusc shells, microfossils, plant remains and infrequent vivianite concretions complete the sediment package of Potrok Aike. The cold conditions allow the precipitation of ikaite, a warmth-sensitive carbonate mineral, on objects like submerged plants and mooring lines. Sedimentation rates during the past 60,000 years were remarkably constant at, with higher sedimentation rates before that and no evidence of gaps in the record. The sediments bear traces of frequent redeposition and mass flows.
Ninety–four volcanic ash layers have been emplaced in Potrok Aike during the past 80,000 years, mostly by volcanoes of the Andean Austral Volcanic Zone such as Aguilera, Lautaro/Viedma, Monte Burney and Reclus. Monte Burney is the main source of tephra at Potrok Aike. Cerro Hudson in the Southern Volcanic Zone also emplaced one tephra during the H1 eruption; its H2 eruption has not been identified at Potrok Aike. No tephra deposits have been attributed to the Pali-Aike volcanic field, probably because it is downwind from the lake. Whether peaks in tephra deposition between 72,000–38,000 and 25,000–19,000 years ago reflect an increase of volcanic activity or increased tephra arrival at Potrok Aike is unclear; for Hudson tephras decreased arrival is the likely cause. Some of the eruptions, in particular the late glacial Monte Burney eruption, severely impacted the lake's surroundings. Several tephra layers were remobilized on land or reworked after deposition in the lake, and thus appear as multiple layers,

Geology and geological history

Potrok Aike is a maar that formed 770,000 ± 240,000 years ago during a phreatomagmatic eruption, when ascending magma interacted with groundwater. Intense explosions yielded a deep diatreme, now partly filled by the sediments on the floor of Potrok Aike. Phreatomagmatic deposits of the eruption crop out east and southeast of Potrok Aike, and in gullies. The intersection of two tectonic lineaments close to Potrok Aike may have directed magma ascent there, and plentiful groundwater in the area facilitated the development of a maar. If the volcanic eruption took place during a glaciation, magma–ice interaction might be responsible for Potrok Aike's large size. After its formation, wave erosion and gravitational slumping would have expanded the water surface, explaining why Potrok Aike has no tephra ring and why it is one of the largest maars in the world.
It is part of the Pali-Aike volcanic field in the back-arc of the Andes. Other volcanic landforms around Potrok Aike are three vents northwest of the lake, an older, 1.19 million years old basaltic lava flow at the southwestern shore of the lake, the Sombrero Mexicano and Hito XXII cones also southeast of Potrok Aike, and the Miocene "Bella Vista Basalts" west of the lake. Potrok Aike is in the older, western portion of the Pali-Aike volcanic field; more recent volcanic activity took place in its eastern parts 10,000 years ago. Volcanic rocks erupted by Potrok Aike contain peridotite xenoliths derived from the mantle, whose formation began during the Paleoproterozoic 2.5 billion years ago.

Geological history

Before Potrok Aike formed, the various Plio-Pleistocene Patagonian glaciers spread into the area from the west and the south and left moraines and glacial sediments. The sediments deposited by the largest of these glaciations have been named "Potrok Aike Drift" or "Sierra de los Frailes Drift" and were emplaced by a glacier propagating from the Seno Otway. The youngest glacial deposits are 760,000 years old and belong to the Río Ci Aike/Cabo Vírgenes glaciations; later advances, such as the last glacial maximum Llanquihue Glaciation did not reach the area and thus did not fill the maar in.
Potrok Aike might have dried up before 53,500 years ago during marine isotope stage 4, perhaps allowing wind to remove lakefloor sediments and form dunes on the lake floor. The lake was continuously water filled during the past 45,000 years, frequently reaching a level of above present-day, where it would overflow to the northwest into the Río Gallegos via a northward flowing drainage named Chorrillo Carlota creek or Río Robes. Overflow was active between 49,000–44,000 and 34,000–17,000 years ago. Between 34,000–44,000 years ago, lake levels decreased and its biological productivity increased perhaps concomitant to warming in Antarctica. Water levels dropped 9,600–9,300 years ago during the Holocene as winds increased and remained below the present-day level. Beginning 7,000 years ago, water levels increased again, with cold and wet periods occurring 4,800, 3,900–3,700, 3,000 and 2,500 years ago. The past two millennia were highly dynamic, with a lowstand during the Medieval Climate Anomaly that correlates with a dry period in the North American Great Basin, overflow during the Little Ice Age, and another decrease since its end.
Other events in its history include:
  • The warming and retreat of glaciers after the last glacial maximum was uneven, with slowdowns and speedups. The Antarctic Cold Reversal is noted by a cooling of about at Potrok Aike, while the Younger Dryas does not appear in temperature reconstructions but appears in the sediments. The sediments also record the A2 and A1 warmings in Antarctica between 36,000–48,000 years ago.
  • Wind speeds were low during the last glacial maximum, then increased during the late glacial in the Bolling-Allerod. During the early Holocene winds were weaker, then strengthened again during the middle and late Holocene. During the Little Ice Age, winds decreased at Potrok Aike. The position and strength of the Southern Hemisphere westerlies is governed by changes in insolation and the intertropical convergence zone.
  • The main sources of lake sediments have varied over time: Outside input dominated between 11,500–9,600 and before 13,500 years ago, and carbonate sedimentation decreased 6,900 years ago and paused altogether during the Little Ice Age.
  • Organic matter accumulation increased during interglacial stages, like between 50,000–49,000, 47,800–45,000, 39,200–36,500, 17,200–16,000, 15,000–14,400 years ago, when increased precipitation, warmer surface water and increased nutrient input stimulated growth in the lake, while the opposite changes occur during glacial periods. Runoff from remnant permafrost contributed to the late glacial peak in productivity. An exception is during 16,000–14,000, when Potrok Aike was impacted by a major eruption of Monte Burney volcano. Oxygen concentrations have been stable for the past 51,200 years.
  • Dust deposition records at Potrok Aike resemble these in Antarctica, reflecting the common source regions. Dust deposition decreased during Heinrich event 1 in both Potrok Aike and elsewhere in the Southern Hemisphere, probably due to a wetter atmosphere washing out dust before it can reach these places.
  • The sediments bear traces of secular variation of Earth's magnetic field, including several Holocene changes in inclination and intensity that have been recorded at other sites around the world. Excursions noted at Potrok Aike are the Laschamp excursion 40,700 ± 1,000 years ago, the Mono Lake excursion 32,400 ± 300 years ago and two changes 46,000 and 20,000 years ago. The latter of which may correspond to the so-called "Hilina Pali excursion". Such records could be used to calibrate paleomagnetic dating.
  • About ten mass movements have been identified in Potrok Aike with magnetic techniques, appearing to correspond to episodes of climate warming, while other Holocene deposits coincide with lake level drops. Some subaqueous mass flows were originally attributed to ancient earthquakes, but most were probably caused by rapid drops of water levels which destabilized the sediments. Increased gustiness may play a role as well.
  • Pollen data indicate that Andean forests persisted in small areas during the last ice age, and expanded again after it ended. During the Holocene, pollen changes reflect moisture or wind changes. Beginning in the 19th century, pollen of introduced plants appears in Potrok Aike, concomitant with the European expansion in the region.
  • Charcoal records at Potrok Aike show wildfire activity both in the Andes and around the lake. They indicate a peak in steppe fire activity between 13,000–11,000 years ago, followed by stable conditions. A fire event around 1600 AD was possibly caused by drought in the Andes.