Migration of the Siple Dome ice divide, West Antarctica

Abstract

ABSTRACT. The non-linea rit y of the ice-flow la w o r a loca l acc umula tion low over an ice di vide can cause isochrones (inte rnal layers) to b e shallower under the divide relative La the fl a nks, forming a "di vide bump" in the intern a l laye r pat tern . Thi s di vide sign ature is ana lyzed using ice-flow models a nd inverse tech niques to detec t a nd q u a ntif y motion o f th e Siple Dome ice divide, Wes t Antarc tica. The principal feature indicating that migrati o n h as occ urred is a distinc t tilt of th e axi s of the peaks of th e wa rped inte rna l laye rs beneath th e di v ide. The calcula ted migration ra te is 0.05-0.50 J11 a I towa rd Ice Stream D a nd dep end s slightl y on wh eth er the di vide bump is caused by the non-linearit y o f ice now or by a loca l acc umul ati on low. Our calcul a tion s also suggest a stro ng so uth-north accumul ati o n g r adient of 5-10 x 10-6 a I in a na rro w zon e north of th e di vide. A co n equ ence of di v ide mig ration is th a t pre-Holocen e ice is thickest abo ut 0.5 km south of the present di vide p osition. Divide m o ti on indicates that n o n-steady processes, poss ibl y assoc ia ted with ac tivit y o f the bounding ice streams, a re a flecting th e geom e tr y o f Siplc D om e. The migrati on r a te is suffi cientl y slow that the di vide bump is maintain e d in th e intern a l laye r pattern a t a ll obse rvable d epth s. This suggests tha t m aj or asy nchrono us cha nges in th e eleva ti on or p os ition of the bounding ice stream s a r e unlikely ove r a t least the pas t 10 3 -10 I yea rs. ( r(z ) ::::(x),lieal1il1gAcc umul ati on rat(' Length sca les for pa rtiti oning fun cti on </ J(x) Di\' ick mi gration rate One less than num be r of free pa rameters Hori zonta l n :locit y Vertical ve loc it y Di sta nce a long fl ow from di vide Number of points along a layer Shape of a laye r with ave rage height E 10tal number of sa mple points Expec ted co mbined error in laye r shapes Wave length of cosine cur ve Di\'ide/Oa nk flow-partiti oning fun cti on Res idual weighting fun cti o n Hori zo nt a l velocilY shape fun cti on Di vide shape fun cti on Fl ank shape fun cti on Integral ove r ice thi ckn ess H of shape fun cti on ( The sta bilit y of th e ' Ve. t Anta rcti c ice sh eet (\\,AIS ) h as b ee n in qu estion since the ea rly 19 705 wh en Wee rlman (1974) m a de the a rg um e nt th at ice sh ee ts g rounded bel o w sea le ve l a re un sta bl e a nd co uld coll a p se catastrophicall y in r es p o nse to a m od est inc rease in sea le ve l. Ch a rac te ri z ing the present a nd future sta bilit y of th e "VAlS is compli cated by th e presence of seve r a l ice stream s. Their capac ity to tra n s p o rt m ass ra pidl y from th e interi o r to the sea p oss ibly inc r eases the potentia l for rapid co ll a pse o f the WAIS. Unde rsta nding th e p as t b eha\'ior of these ice stream s sh o uld prov ide importa nt clues to th eir rol e in '''' IS stability.Inte r-i ce-stream rid es are good pl aces to look fo r ev id e n ce of pas t ice...

Dating ice flow change near the flow divide at Roosevelt Island, Antarctica, by using a thermomechanical model to predict radar stratigraphy

“…Since these produce bump amplitudes that are too small, we need to seek a meteorological mechanism that increases the amplitude. Scouring at the divide could be such a mechanism, as was pointed out by Fisher et al [1983] and later analyzed by, for example, Nereson et al [1998b] and Nereson and Waddington [2002].…”

“…The initiation of the divide flow resulted in the formation of Raymond bumps. These are convex arches in the layers seen in radar profiles obtained across some ice divides [e.g., Nereson et al, 1998b;Vaughan et al, 1999;Conway et al, 1999]. These layers are believed to be syndepositional and are therefore termed isochrones.…”

“…Knowledge of ice rheology is key to matching any type of ice model to data and to making predictions of the way glaciers will behave in the future. It is known that wind scouring at the divide could enhance the Raymond effect [e.g., Nereson et al, 1998b;Nereson and Waddington, 2002], so we also consider its effects. Matching of the observed asymmetry of Roosevelt Island surface profile posed special difficulties, and we will test asymmetric accumulation rate as a possible reason for this surface profile asymmetry.…”

On the effects of anisotropic rheology on ice flow, internal structure, and the age‐depth relationship at ice divides

“…This stiff zone of ice leads to the formation of anticlines, i.e., an upward wrapping of isochrones. These anticlines, now generally referred to as Raymond bumps, were subsequently found in radargrams collected at ice divides [e.g., Nereson et al, 1998b;Vaughan et al, 1999;Conway et al, 1999]. In radargrams, a vertical series of Raymond bumps are commonly seen, with one Raymond bump on the top of another forming a Raymond stack.…”

Constraining Geothermal Flux at Coastal Domes of the Ross Ice Sheet, Antarctica

“…Siple Dome has been investigated previously, and an ice core has been drilled to bedrock (Nereson et al, 1998;Engelhardt et al, 2004;Taylor et al, 2004). Borehole temperatures have been measured with thermistor strings frozen into the ice column (Engelhardt, 2004) and in the fluid-filled ice-core borehole (Gary Clow, reported in MacGregor et al, 2007).…”