Lake Eyre (Kati Thanda), Australia
Onset of formation of an ephemeral salt crust about the edges of the lake.
The mean annual runoff across the basin is 4 km3, equivalent to a water layer 3.5 mm deep; this is the lowest runoff level of any major drainage system globally. The Lake Eyre Basin is vast but dry. Its drainage spans several climatic zones, including the southern edge of the tropical monsoon system to the north and the mid-latitude westerly circulation belt to the south. Some 5 x 105 km2 of the drainage basin receives less than 150 mm of rainfall annually (on average). The highest precipitation, with annual averages around 400 mm, occurs in the northern and eastern margins, where moisture comes from the monsoon belt's edge. Water from the monsoon belt enters every few years and can take two to ten months to reach the lake sump. Even in the dry times, water usually remains in some of the 200 smaller sub-lakes surrounding the lake.
During lake-full stages, the feast of phytoplankton and zooplankton attracts many aquatic birds to feed in the mesohaline waters and nest about the lake edges. During the 1989–1990 flood, 200,000 pelicans, 80% of Australia's total pelican population, came to feed & roost in the flooded lake. Two zooplanktonic species, Ammonia beccarii and Elphidium crispum, are carried in by the migrating seabirds and, if salinities are suitable, will flourish for short periods. Such encysted marine forams are carried hundreds of kilometres inland on the feet and in the feathers of the coastal sea birds. If surface water salinities are suitable, the forams bloom. Later, as salinities in these inland lakes increase, the foram population dies back, and tests are deposited in a lacustrine sediment matrix. Such avian transport and seeding can explain marine microfossils in lacustrine sediments.
Lake Eyre, South Australia, pre- and post-flood. The image on the left was taken on January 1, 2000, before a significant flooding event. The partly cloud-obscured image on the right taken on April 6, 2000, after a significant monsoon-fed flood influx into the lake from its northern part. The elongate interdunal sabkhas and pans in the northern part of the image constitute the Kallakoopah Pans. Note the water-filled Warburton Channel is transecting the northern lake floor (Landsat 7 satellite images acquired and processed by ACRES).
Lake Eyre bathymetry, facies and flooding history compiled from Kotwicki (1986), Kotwicki and Allan (1998) and various references cited in text. Lake floor contours are in metres below sea level. Inflow volumes (1885-2012). Current water conditions accessible at http://www.lakeeyreyc.com/Status/latest.html
Locations of spring mounds, spring groups and artesian groundwater flow paths in the Great Artesian Basin, Australia. Shows a high density on spring groups in the vicinity of Lake Eyre and Lake Frome (after Habermehl, 1982). Inset is a idealised schematic cross section.
The northern end of the Lake Eyre catchment lies barely on the periphery of the planetary monsoon system and is subject to year-to-year fluctuation. The Northern Australian monsoon is erratic both in space and time. Recent lake floods are related to La Nina phases of the El Nino-Southern Oscillation (ENSO) and are out of phase with the Pacific Dry Zone rainfall and El Nino events. Changes in the intensity of the monsoon have controlled the frequency and permanency of Lake Eyre waters throughout much of the Quaternary (see Warren, 2016; Chapter 3 for detailed literature compilation of the lake geology and climatology).
Eyre’s position also defines the southern outflow or seepage area of the Great Artesian Basin (GAB), which covers an area of 1.7 million km2. The GAB consists of several contiguous sedimentary basins with confined aquifers of Triassic, Jurassic and Cretaceous continental quartzose sandstones, underlain by an impervious pre-Jurassic base (Habermehl, 1980, 1982, 1996). It forms a sizeable synclinal structure, uplifted and exposed along its eastern margin, leaving the overall basin tilted southwest. Recharge to the GAB occurs primarily along the uplifted eastern margins and the western margins, where the aquifers are exposed or overlain by sandy sediments. Confined groundwater then percolates very slowly towards the terminal base level of Lake Eyre and saline lake sumps, reaching the Lake Eyre area after an estimated travel time of up to three million years.
Spring mounds
Natural discharge from the GAB comes from two fundamental processes: 1) diffuse vertical leakage towards the regional watertable and 2) from focused outflow via 600 springs located in 11 groups along the western and southern boundaries of the Lake Eyre Basin. Many spring mounds define linear fault-controlled surface trends. Flow from the mounds totals 0.03 km3/year and is a minor discharge component of the overall diffuse artesian discharge to the watertable. But it creates distinct carbonate spring mounds with the potential to be preserved (Ponder, 1986; Mudd, 1998, 2000; Keppel et al., 2011). Measured flows from various springs range from 0.0001 to 0.23 m3/s, totalling around 1 m3/s. Salinities range from 700 to 80,0000 ppm, pH from 7.1 to 8.0 and water temperatures from 30 to 40°C.
The Lake Eyre basin has been an area of lacustral sedimentation since the Mesozoic. The first documented lacustrine sediments are of freshwater Lake Walloon, which occupied the central-eastern part of the Australian continent some 150 Ma (Middle Jurassic). By the late Cretaceous (85-75 Ma), it had shrunk to become the much smaller 5 x 105 km2 Lake Winton. By the early Miocene (21 Ma), the freshwater lake had assumed a position closely resembling Lake Eyre's; it even had an indigenous species of freshwater dolphin. This freshwater system, in turn, evolved into Lake Dieri, a Pleistocene "greater Lake Eyre or mega-Eyre", which underwent large-scale variations in size and salinity from a perennial freshwater lake up to 25 metres deep to a groundwater-controlled playa marked by substantial sediment deflation to enter the Holocene as a dry salt pan subject to occasional flooding (see Warren, 2016 for detail).
Lake Eyre, Spring Mounds. A) Active mound spring in the southern margin of the Great Artesian Basin artesian where water up to 3 million years old returns to the surface. B) Schematic cross section of typical Lake Eyre Basin mound spring.
Arid sediments in the Lake Eyre sump
The most significant aridity in Lake Eyre during the last 150,000 years occurred in the marine oxygen isotope stage (MIS) 6, marked by a 4.3 m basin deflation lower than today (see A in the figure below). This arid event was a stark contrast to the sudden formation of the deepest perennial lake (phase V at +10m relative to the Australian height datum [AHD—mean sea level]), a level some 10 metres above sealevel and some 25 metres above the modern playa floor. At that time, at least 6 m of finely laminated gypsiferous and calcareous clay were deposited under salinity stratification and strongly reducing bottom conditions (Magee et al., 2004). Simultaneously, meandering inflowing streams deposited thick, lateral-accretion sediments and fluviodeltaic sediment at the stream mouths.
Following this acme level, the lake experienced a brief drying period, but no evidence of deflation or pedogenesis was observed. Between 100 and 75 ka (Phase IV), the lake refilled to +5 m relative to AHD, and deep-water lacustrine clays, calcareous nearshore and beach sands, and clastic gypsum evaporites were deposited coevally with renewed fluvial aggradation.
Fluctuations in lake level and salinity suggest a decreasing regularity of inflow and gradual diminution of the monsoon toward the end of Phase IV. Phase IV sediments were pedogenically modified and eventually truncated by deflation when the lake dried at 75–70 ka and disrupted gypsiferous playa sediments were transported downwind. The incision of tributary rivers into previous fluvial and lacustrine sediments, extending almost to the current level, documents a transition to significant aridity. Lacustrine conditions returned at 65–60 ka (phase III), depositing lake sediment and a prominent beach sand rich in the brackish water gastropod Coxiellada gilesii at -3 m relative to AHD; coeval fluvial aggradation and vertical accretion of overbank muds occurred. Though having shallower water and shorter duration than Phases V and IV, Phase III had significantly deeper water and longer duration than the modern ephemeral events and represents the last deep-water perennial lake in the basin and the last influence of moderately effective monsoon precipitation. A major deflation episode between 60 and 50 ka excavated the present Lake Eyre basin to deposit gypsum- and clay-rich lunettes at several sites around the lake edge, as well as covering former shorelines on the windward side of the lake. After deflation ceased, a thick secondary gypsum soil profile (gypsite) developed on the dunes early in oxygen-isotope stage 3, between 30,000 and 12,000 yr. B.P. Lake Eyre was at least as dry as it is today. A lunette-like, playa-marginal, eolian unit formed from saline mudflat sediments deflated from the playa floor at many sites. After 10,000 yr B.P., a minor lacustral phase occurred (Phase I) until the current ephemeral playa regime became established at 3000-4000 yr B.P.
Lake Eyre stratigraphy. A) Late Quaternary changes in lake level in Lake Eyre. B) Distribution of buried saline pan halite beds and associated sediments in Madigan Gulf, Lake Eyre North.
Given the large volumes of water periodically entering the Lake Eyre depression and its long history extending back to the Mesozoic, one would perhaps expect large quantities of salt to have, at times, accumulated in the lake sediments. Indeed, Lake Eyre (Madigan Gulf) is the only Australian playa that retains buried Pleistocene halite beds (B in Figure above), but both beds are less than a metre thick. The total Pleistocene thickness is only 3 metres in the saltiest part of Lake Eyre North. A lack of preservation/accommodation space, tied to ongoing lake floor deflation, reflects the meteoric/artesian recycling hydrology that also characterises most other large low relief playas of inland Australia (Chapter 2; Warren 2016). The buried halite in Madigan Gulf is a former salt-pan unit, which was partially dissolved during the onset of a minor halite phase in the early Holocene and then preserved by the deposition of the overlying gypsiferous clays. However, it is not a geologically significant halite body with thicknesses of less than a metre for each of the two preserved saline pan beds. Inherent low relief and artesian hydrologies mean the ephemeral stream floodplain – dune field – ephemeral saline lake playas of Australia and elsewhere cannot accumulate thick sequences of bedded salt in the lowest parts of the drainage basin. Salts and other saline siliciclastic lacustrine sediment blow away. Haloturbation of siliciclastics in continental sabkhas associated with widespread redbed fluvial and eolian deposition is the dominant signature of this deposition style, not stacked halite beds.