46th Lunar and Planetary Science Conference (2015)
2016.pdf
THE MARTIAN PARAGLACIAL PERIOD AND IMPLICATIONS FOR LATE AMAZONIAN CLIMATE.
E. R. Jawin1, J. W. Head1, and D. R. Marchant2, 1Department of Earth, Environmental, and Planetary Sciences,
Brown University, Providence, RI 02912 USA, 2Department of Earth & Environment, Boston University, Boston,
MA 02215 USA (Erica_Jawin@brown.edu).
Introduction: In terrestrial glaciated environments, a short period of modification has been identified immediately following a period of deglaciaion
which represents the environmental response to ice
loss. This paraglacial period is characterized by accelerated sediment transport rates and modification, and
ends once these rates return to non-glacial or equilibrium conditions [1–3]. The paraglacial period is distinct
from periglacial processes and features which occur in
cold, nonglacial environments or at glacial margins.
Late Amazonian glacial periods on Mars have been
observed, attributed to orbital forcing and obliquity
variations [4,5], which are characterized by ice migration from the poles to lower latitudes and back [5–7].
In support of this theory, ice-related features have been
documented in the mid-latitudes in the form of concentric crater fill (CCF), lineated valley fill (LVF), lobate
debris aprons (LDA), and tropical mountain glaciers
[8–10]. As much of Mars experienced periods of glaciation, it is logical to also expect periods of deglaciation. Therefore, it is hypothesized that a martian paraglacial period also exists, representing the transition
from a glacial to a post-glacial climate [11,12].
To understand the global effects of glaciation and
deglaciation, craters bearing CCF were analyzed
across the mid-latitudes for evidence of ice loss and
paraglacial features [12]. Understanding the initiation
and duration of the martian paraglacial period will aid
in the understanding of the Late Amazonian climate
variation, as well as climatic patterns of ancient Mars.
The Paraglacial Period: On Earth, a suite of
“land systems” has been identified which comprise a
range of processes and features typical of the paraglacial period: 1) slope evolution, characterized by
sackungen (uphill-facing scarps), 2) sediment modification by fluvial and aeolian activity, characterized by
gullies, 3) thermal cycling, characterized by gelifluction/solifluction lobes and polygons, and three additional land systems (alluvial, lacustrine, and coastal
systems) which are unlikely to be present on Amazonian Mars [2]. It is the combination of these separate
land systems that describes the paraglacial period, as
many features typical of this period can form in nonglacial environments; as such, the paraglacial period is
viewed as a combination of features and processes
representing the broad scale environmental response to
deglaciation (Fig. 1) [1,2,13,14]. Jawin et al. [11,12]
have described the individual terrestrial land systems
and components and their application to the martian
paraglacial period, including the morphologic features
spatulate depressions, gullies, washboard terrain, and
polygons.
Martian Paraglacial Period and Climate: The
martian craters analyzed in this study which contain
paraglacial features also contain relict glacial features
in the form of CCF, which implicates a period of glaciation in the relatively recent history of Amazonian
Mars. This glacial ice accumulated through the deposition and eventual mobilization of snow and ice in the
Figure 1. Chronology of the terrestrial paraglacial period. (A)
Glacier retreat triggers mass wasting and gully incision. (B)
Glacier continues to retreat, gullies incise, and glacial forelands
are modified. (C) Paraglacial period ends when sediment
transport rates return to non-glacial levels and gullies and debris
cones are stable, often due to vegetation. Modified from [14].
46th Lunar and Planetary Science Conference (2015)
gully alcoves of these craters [15]. The glacial period
is expected to have persisted for up to tens of millions
of years, in a period of elevated obliquity where the
mean planetary tilt was ~35° (Fig. 2) [16].
At approximately 5 Ma, the mean obliquity decreases from 35° for a period of ~2.5 million years,
until it reaches a relatively stable minimum mean
obliquity of 25°, which has persisted from 2.5 Ma until
the present. During the period of decreasing obliquity,
the rate of snow and ice deposition would slow and
eventually halt. In addition, any exposed snow and ice
would sublimate, causing glaciers to retreat. This represents the transition of the environment from a net
accumulation to a net ablation period (via sublimation
rather than melting). This transition is visible in the
form of spatulate depressions inside the martian craters; the location, orientation, and stratigraphic age of
these depressions suggests that they formed from the
recession and/or sublimation of large lobes of glacial
ice originating from the alcoves on crater walls. The
shape of the spatulate depressions mirrors the shape of
the concentric ridges in the CCF on crater floors, and
the depressions are stratigraphically younger than the
CCF but older than the paraglacial features. These
characteristics suggest the spatulate depressions
formed after the ice was deposited on the crater floors,
but before the paraglacial features had formed, in a
period of decreasing obliquity and net ice loss (Fig. 2).
The paraglacial features are stratigraphically
younger than the spatulate depressions, and are expected to have formed between 0.5-5 Ma, during the
period of decreased mean obliquity (Fig. 2), due to
instabilities in the crater caused by the removal of ice
from the crater floor and walls [2,12]. Several paraglacial features, namely gullies and washboard terrain,
2016.pdf
are associated with alcoves which represent the peak
accumulation areas in glacial periods. The formation
of gullies and washboard terrain is spatially and temporally related, forming throughout the paraglacial
period and aided by small amounts of liquid water.
In terrestrial environments, the paraglacial period
ends and a post-glacial climate is reached when sediment transport rates reach non-glacial conditions.
However, this is an impractical metric on Mars due to
the low erosion rates and lack of abundant liquid water
in the late Amazonian. However, the formation of gullies and washboard terrain appears to have slowed or
halted, and the dominant modification process in the
martian mid-latitudes currently appears to be eolian. It
may be suggested that the current (~0.5 Ma – presentday) climate on Mars is that of a post-glacial setting.
Analyses of craters bearing CCF and associated
paraglacial features across the mid-latitudes have
shown that paraglacial reworking is seen across the
planet in both hemispheres. This widespread distribution suggests that the paraglacial period is a large-scale
process that persists for a substantial period of time.
References: 1. Church and Ryder, Geol. Soc. Am. Bull. 83,
3059–72 (1972). 2. Ballantyne, Quat. Sci. Rev. 21, 1935–
2017 (2002). 3. Ballantyne, The Holocene. 12, 371–6 (2002).
4. Laskar et al., Icarus. 170, 343–64 (2004). 5. Head et al.,
Nature. 426, 797–802 (2003). 6. Forget et al., Science. 311,
368–71 (2006). 7. Madeleine et al., Icarus. 203, 390–405
(2009). 8. Squyres, JGR. 84, 8087–96 (1979). 9. Baker et al.,
Icarus. 207, 186–209 (2010). 10. Head and Marchant, Geology. 31, 641–4 (2003). 11. Jawin et al., LPSC, 45th (2014),
Abs. 2218. 12. Jawin et al., LPSC 45th, (2014) Abs. 2241. 13.
Eyles, S.P. Kocsis, Sediment. Geol. 65, 197–8 (1989). 14.
Ballantyne and Benn, in Adv. Hill. Proc. (1996), vol. 2, pp.
1173–95. 15. Fastook and Head, PSS Sci. 91, 60–76. 16.
Schon and Head, EPSL. 317–318, 68–75 (2012).
Figure 2. Martian obliquity variations modeled by [4] for the past 14 million years. Periods of elevated obliquity (~35° mean
obliquity) represent the glacial periods which led to formation of current remnant glacial features (CCF, LVF, LDA). As mean
obliquity decreased, surface ice became unstable and was lost, triggering the paraglacial period ~5 Ma, which persisted through
the most recent ice age outlined by [5]. The current climate is expected to be a post-glacial climate, with eolian reworking as
the dominant form of modification. Modified from [16].