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Sedimentological discoveries in enigmatic 'conglomerate platform' on Pacific atolls
support Holocene rapid meter-scale sea-level oscillation
Roger Higgs DPhil, Geoclastica Ltd, UK Geological Society of America Annual Meeting, Denver, 2022
Mili Atoll (20km across), Marshall Islands, latitude 6oN, Pacific Ocean
Photo: Roger Higgs 2019
Abstract
https://www.nationsonline.org/oneworld/map/oceania_map.htm
Majuro
Location of 5 mentioned atolls
Tarawa
Caroline
Fakarava
1,000 km
Hawaiian Islands
Takapoto
Typical atoll
ATO LL
RIM
Patch reef
https://sites.google.com/a/markham.edu.pe/mr-
carter-s-igcse-geography/oceans/coral-reefs
Islet Atoll rim, hosting 'conglomerate
platform' (CP, not shown)
Coral
growth
Coral
growth
Conglomerate platform (CP): patchy distribution on atoll rim
Ocean
Low
High spring tide
Lagoon
Idealised atoll rim
Woodr offe 2008
ATO LL R IM
Sandy islet, perched on CP
, CP
CP ... nine outcrop characteristics
1. Comprises cemented coral rubble
2. Surfaces are brown or black
(boring microbes);
interior white (coral)
cont'd ...
Ocean
Islet
CP = cemented
angular coral
rubble Pack
40cm
Majuro Atoll
Loose rounded
coral rubble
CP ... outcrop characteristics
3. Patchy distribution
4. c. 1m thick
5. Under water at high spring tide
(except atolls with low tidal range)
6. Overlies 'reef flat' *
cont'd ...
Ocean
Man
Reef flat
(exposed at low
spring tide)
CP edge = scarp
retreating by
wave erosion
Tara wa Ato ll
Eroded CP block,
c.1m across
*Reef flat = dead coral, c.4-2kyBP. Likely interpretation: highstand reef, shaved flat by waves under falling sea level (SL), prior to CP deposition
CP ... outcrop characteristics
7. Cross bedded. Previously unreported? Single set of foresets. Formed by avalanching at front of advancing rubble 'bar'? Single event?
Reef flat
Ocean
CP upper surface (see plan view, Photo 2)
CP scarp, c.1m,
erosional
40cm backpack
Tara wa Ato ll
12
Ocean
CP
upper
surface
cont'd ...
Foresets, single set,
dipping oceanward
CP ...outcrop characteristics
8. Bordered by an eroding scarp
9. Pervasive karstic solution pitting
8, 9 prove CP out of equilibrium in
present-day intertidal setting
Suggests subtidal deposition at
higher SL
Later emergence by later SL fall
Tarawa
CP scarp,
c.1m,
erosional
Perched islet
Reef flat
(mid-tide)
Lagoon beyond trees
CP upper surface
Pervasive karstic pitting
People
CP characteristics on satellite images
(next 4 slides)
-patchy
-individual CP patches are tulip-shaped, comprising a 'stem' & 'flower' ... not previously reported
-'flower' faces ocean
-'flower' oceanward edge is 'frayed' (erosion)
-'stems' & 'flowers' show 'nested crescents' (curved foresets)
-crescents can be concave-oceanward or -lagoonward
Majuro, CP 'tulips' on atoll rim
1. Array of oceanward-flaring CP 'tulips' (brown)
2. Many tulips have a perched sandy islet (white; +/-green trees)
3. Many tulips coincide with notches in atoll rim inner edge
OCEAN
LAGOON
notch
perched islet
'stem'
'tulip'
notch
NB:
'flower'
10km
CP 'tulips', north Tarawa
fish
trap
'Tulips' protruding
from beneath
sandy islet'
'Tulip' frontal erosion
& 'nested crescents',
concave lagoonward
OCEAN
CP 'tulips', south Tarawa
Islet
perched
on CP
'flower'
Reef flat
OCEAN
LAGOON
CP oceanward
margin
'frayed' by
erosion
Reef flat,
sand-draped
Crescents,
concave-
lagoonward
Crescents,
concave-
oceanward
'Stems'
CP 'tulips', Fakarava Atoll
Lagoon
Ocean
Nested
crescents,
concave
oceanward
'Flower'
Islet
'Stem'
Two
overlapping
'flowers'
'Stem'
600m
200m
CP coral-clast ages
Mostly 6.5 -2.5kyBP (literature survey)
Dated coral clasts in CP scarp
Montaggioni et al. 2021
Lagoon
Ocean
e.g. Takapo to Atoll
CP usual interpretation ...
Inward transport (from ocean-side reef front, by storms)
CP new interpretation ...
Outward transport (from lagoon reefs). Evidence ...
1. 'tulips' dilate oceanward, i.e. 'stem' expands into 'flower'
2. foresets dip oceanward
3. 'flowers' never reach outer (oceanward) edge of atoll rim
4. 'stems' commonly meet inner (lagoonal) edge of atoll rim and
5. commonly coincide there with notches (feeder conduits?)
6. CP can cap intra-lagoon patch reefs (Montaggioni et al. 2021)
Amalgamated CP 'tulips'
Lagoon patch-reef network
notches
notch
Tulip oceanward edge 'frayed' by erosion
Overlapping
'flowers'
New model: CP coral rubble (brown) bilaterally ejected from lagoon
Caroline Atoll
Vegetated
islet atop CP
How ? ... next slide
Ocean
Classic
'tulip'
Classic
'tulip'
I propose outward transport of coral rubble, by tsunami
Published computer-& lab modeling suggest 2 effects of a tsunami entering a lagoon:
1. vortices (capable of throwing coral rubble into suspension?)
2. rise in water level, hence outward pressure gradient (coral transport?)
... see next 2 slides
Boshenyatov & Zhiltsov 2019
'Vortex suppression of tsunami-like
waves by underwater barriers'
Ocean Engineering, 183, 398-408
Chain of counter-rotating
stationary vortices
Laboratory flume tank
(simulates lagoon)
Time elapsed =
5 seconds
Simulated
tsunami wave
50 cm
Flume barrier
(atoll-rim analog)
Tsunami effect 1 ... lagoon vortices
10
cm
Rasheed et al. 2022
Model of 2004 Boxing Day
tsunami, Indian Ocean
Bathymetry refracts
tsunami into a
'bow wave' (red)
25 km
tsunami
amplitude,
m
propagation
Tsunami effect 2 ... band of higher water (tsunami wave) traversing lagoon ...
Lagoon
Indian
Ocean
Maldives
... must cause bilateral outward pressure (black arrows);
likely multiple pulses due to tsunami wave-train
When was the tsunami. ?
c. 2.5kyBP (very approximate),
based on CP coral-clast age span,
6.5 -2.5kyBP
NB Dated coral clasts are entirely younger in some CPs (Montaggioni et al. 2021),
suggesting a younger tsunami (consistent with overlapping 'flowers')
Was SL higher during CP deposition?
CP disequilibrium (erosion, dissolution) in present intertidal setting ...
... suggests deposition below low tide ...
... implies SL at least 1m higher than today ...
... confirmed by ...
... two independent SL curves showing SL c.2m higher than present at c.2.5kyBP ...
2. SL for 5 Pacific atolls
(in Kiribati & Tuvalu)
Based on C14-dated giant clams
'Abrolhos Transgression' c.2.7kyBP.
rapid (c.100y) 5m rise, reaching +2m
'Gilbert-III' Transgression,
rapid 4m rise, reaching +2m
Fair agreement between independent SL curves, last 4ky...
5m
Present
sea level
4kyBP 0 kyBP
1,000y
3ky 2ky 1ky
1. World SL
Based on diverse SL benchmarks
around the world
Famous 'Fairbridge curve' (1961)
Schofield 1977
'Florida
Emergence'
'Rottnest
Transgression'
NB C14 dating loose,
typically +/-100-200y
Proposed atoll history since 3kyBP ...
1. c.2.7kyBP, SL rapidly rose c.5m in c.100y (cf. proven 3m SL rise in decades, previous interglacial) ...
average 5cm/y ... outpaced coral growth, so water deepened.
Fairbridge 'Abrolhos transgression'
2. During highstand (at +2m), CP was deposited below low-tide by a tsunami, c. 2.5kyBP
3. SL fell c.5m by 1.7kyBP ... 'Florida Emergence'. Benthic-foram sand on atoll rim & lagoon fringe
emerged subaerially & was heaped into eolian dunefields
4. SL rose c.3m by 1.0kyBP ... 'Rottnest Submergence' (abundant evidence in UK Roman-& Norman-age
seaforts & waterfronts, meticulously excavated & tightly dated). Atoll eolian dunefields shrank by wave
attack (ravinement), forming today's remnant sandy islets.
0.5m
Conflicting Holocene SL reconstruction -said to constrain oscillations to <50cm
(accepted by IPCC)
BUT note:
-sample clustering
-wide error bars
permitting multi-century data gaps
Potential data gaps 'hiding'...
-highstands (too deep for microatolls to form)
-lowstands (microatolls unseen/unsampled,
drowned 2-3m below present low-tide)
Woodroffe et al. 2012
Kiritimati Island (mid-Pacific). Based on coral microatolls (v shallow water), dated mostly by C14.
cluster cluster clustercluster
Sun's magnetic output
From ice-core Be10 & tree-ring C14
High resolution
Wu et al. 2018
-1.5
0
1.5
3
010002000300040005000
World SL
From diverse SL benchmarks worldwide
Fairbridge & Hillaire-Marcel 1977
Greenland air temp.
From ice-core isotopes
High resolution
Vinther et al. 2009
Abrolhos transgression
Partial
correlation ?
BC AD
Solar origin of Fairbridge SL oscillations ?
Te mp .
deviation from
smoothed
estimate of
present temp.,
℃
Reconstructed
sunspot
number
Slant = 100-150y lag
5m SL
today
No lag? (SL dates loose, C14)
2000 BC 2000 AD
NB
Temp .
lag
Some tentative conclusions
Multi-meter SL oscillations occurred in Holocene ...
... driven by temperature changes, possibly caused by solar fluctuations (c.100-150y lag)
cf. Svensmark theory (cosmic ray/cloudiness link)
OMINOUSLY ...
Sun's magnetic output surged 1900-1991, to highest value in >4ky (previous slide, red dashed line at top)
Portends Sun-driven warming & another c.5m SL rise, peaking no later than c.2140 (assuming 150y lag) ...
... brought forward & exacerbated by ongoing man-made warming? (see my GSA 2022 virtual poster, Tuesday)
cf. IPCC 'worst case', only 1m SL rise by 2100