Desert belts merge in Pangaean time

Generally, the world’s Phanerozoic climate is divided into greenhouse (no permanent polar ice) and icehouse (permanent polar ice cap(s)) periods. Higher CO2 and higher temperatures typify greenhouse.
Across the Phanerozoic, in both climate settings, larger deserts were generally tied to climate belts maintained by sub-tropical (Hadley Cell) positions and separated by a tropical equatorial belt.

That is, much of the world-scale distribution of significant bedded evaporite accumulations across the recent and the Tertiary indicates the ongoing presence of two mid-latitude arid belts centred on Hadley There are exceptions in locales with belts extending further north or south in regions generated by local rain shadows and orographic control provided by neighbouring mountain ranges. But at the time of Pangaea and its initial break-up (mid-Permian to Jurassic), when the world had a single landmass (supercontinent) with large continental areas straddling the equator, the evaporite distribution does not separate in two belts. Instead, the two arid latitudinal belts extend further into equatorial zones. It seems the Pangean supercontinent had a single arid to hyperarid zone spanning the Pangean continental interior and the equator.

This is seen below in two reconstructions of the Pangaean supercontinent, the first some 290 Ma, shows ice sheets at the southern pole and a well-developed tropical corridor indicated by climate-sensitive deposits (bauxites and coals) separating the two arid subtropical (Hadley cell) regions. The second at 255 Ma shows an equator-spanning desert belt.

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Relationship between icehouse and greenhouse periods, eustacy and global CO2, along with the latitudinal extent of glacial tillite. Warm greenhouse conditions prevail over much of the Phanerozoic.

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 Latitudinal distribution of evaporites from Permian to present (after Ziegler et al., 2003). Shaded belts (rectangles) indicate horse latitudes (Hadley cells). Note, this plot is a count of an evaporite occurrence in each one degree latitude-longitude region; it plots latitude occurrence and is not a plot of evaporite volume. The dominance of northern hemisphere occurrences reflects the dominance of landmasses in the northern hemisphere across the Permian to Recent.

This rock-based reconstruction of the mega-monsoonal Pangaea passes from icehouse at 290Ma to greenhouse at 255Ma, shows the lack of tropical sediment indicators outside the region surrounding the Panthalssian ocean located to the east of the central supercontinent landmass. At that time, the interior of the supercontinent was typified by an arid to hyperarid desert belt extending across the equatorial zone. This is a very different equatorial setting compared with the tropical equatorial belts of the Quaternary. In the later Permian, through the Triassic, and much of the Jurassic the two formerly mid-latitude Hadley Cells merged over the more central Pangaeanic regions of Africa, Europe and the adjacent Americas, to form an arid belt that also encompassed low-latitude, arid equatorial areas (Figure 3).

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Permian palaeoclimatic reconstructions of the Pangaean Supercontinent redrafted and replotted from Boucot et al. 2013, and the Scotese PaleoAtlas mounted in GPlates 2.0. A) Reconstruction of Pangaea, some 290 Ma, with ice sheets at the southern Pole and a well-developed tropical corridor indicated by climate-sensitive deposits. B) Reconstruction of Mega-monsoonal Pangaea, some 255 Ma, showing the lack of tropical sediment indicators outside the region surrounding the Panthalssian ocean.

Across the continental interior of the Pangean supercontinent, this arid to hyperarid equatorial belt in the supercontinent interior prevented the formation of climate-sensitive sediments that are more typical of humid equatorial conditions that deposit coals, kaolinites, lateritic materials and bauxites. However, in this same time interval, these more humid sediment products are typically present at low latitudes of these time slices adjacent to the Panthalassic ocean. That is, in the absence of an equator-spanning supercontinent, low latitudes, usually imply humid and non-seasonal tropical conditions throughout much of the Phanerozoic, as we see today. But the assembly of the Pangaean supercontinent disrupted this latitudinally-zoned atmospheric circulation, replacing it with a progressively more monsoonal (seasonal) circulation and more arid, at times hyperarid, conditions in the equatorial continental interior of Pangaea (Parrish, 1993). The Pangean supercontinent reached its maximum areal extent in the Triassic and was associated with what is known as the Pangaean Megamonsoon. There were immense arid regions across the interior parts of the supercontinent that were nearly uninhabitable, with scorching days and frigid nights. However, Panthalassian coasts still experienced seasonality, transitioning from rainy weather in the summer to dry conditions during the winter and the associated accumulation of humid sediments (Figure 3b; Boucot et al., 2013). Megamonsoon aridity is evidenced not just in the accumulation of low-latitude bedded evaporite deposits. Low latitude continental aridity also drove the accumulation of thick, widespread low-latitude desert redbeds, sourced by eolian, not fluvial, detrital transport (Sweet at al., 2013) and the precipitation of bedded salt crusts in ephemeral saline lakes under exceptionally-high surface temperatures.

How hot was it?

Work on the Nippewalla Group, using a halite core recovered from the Amoco Rebecca K. Bounds well, has further refined the climate-temperature significance of brine inclusions in aligned chevrons in bedded halite deposited in what was a continental interior playa system during Pangaean time.

Characteristic lithologies comprise bedded pan evaporites, red-bed siliciclastics and grey siliciclastics, all laid down in a series of continental playa depressions (Figure 5.15b, c). Most evaporite deposition took place in a series of halite-dominated ephemeral saline lakes surrounded by saline and dry mudflats, sandflats and sand dunes. It was an evaporitic environment characterised by acid groundwaters with hydrologies not unlike those of southeastern interior Australia and exemplified by the redbed lacustrine associations forming today in and around Lake Tyrell, Victoria and the salt lake landscape of southwest Western Australia

Extremely high temperatures, ranging to 73°C, and large diurnal temperature ranges are evidenced in brine inclusions in the lower Nippewalla Group, suggesting conditions more extreme than anywhere on Earth today (Zambito and Benison, 2013). In contrast, the upper Nippewalla Group was cooler; maximum temperature was 43°C, and diurnal temperature ranges were smaller, though even these conditions are similar to modern extremely hot environments, such as Death Valley, California. Comparison to prior studies suggests that these temperature results may be indicative of regional patterns.

It seems that the vast supercontinent interior deserts of Pangaea at times experienced more extreme temperatures than today, due to the high degree of continentality in such deserts and the fact that the Neogene Earth climate is in icehouse mode.

Another time of extremely hot climatic conditions occurred in the Aptian (Cretaceous) when tachyhydrite was a primary marine evaporite precipitate (see Salty Matters, February 28, 2017).

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Homogenization temperatures (Th; MAX—maximum, AVG—average) for fluid inclusions within Nippewalla Group halite in the Amoco Rebecca K. Bounds core, Individual values represented by black-edged red circles. Stars represent ThAVG for each sampled bed; ThMAX is traced within subgroup units by solid red line. Thin red dashed line marks 56.7 °C, the hottest known air temperature measured on Earth (Death Valley, California). Inset in upper right plots Thmax versus ThAVG for all beds sampled (after Zambito and Benison, 2013)