The southern North Sea? Rhine-Thames ...

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The southern North Sea? Rhine-Thames land!
Hijma Mare
Deltares, PO Box 85467, 3508 AL Utrecht, the Netherlands
E-mail: m arc.hiim [email protected] deltares.nl
A series o f three papers dealing w ith the closely interlinked topics o f Holocene and Pleistocene
geological evolution o f the Southern North Sea - also dubbed Rhine-Thames land - is presented
here. For th is sum m ary it is best to start w ith the paper th a t appeared last (Hijm a e t a i, 2012),
which deals w ith landscape change in the last m illion years and the w ider Southern North Sea
region. To paleohum ans back then and to us modern man today, the landscape o f the North Sea
region looked vastly d iffe re n t one m illion years ago. A landbridge connected England to Belgium
and the rest o f continental Europe. Since then, however, several cycles o f m elt waters from land ice
have stripped the te rre stria l landscape o f the land bridge, allow ing the sea to enter the area in
interglacials. The paper links Q uaternary Geological research questions on tim in g and rates o f
landbridge low ering and removal to research questions in archaeology, on dispersal o f
paleohum ans fro m refugia and behavioural e volutionary adaptions to living in tem perate habitats.
Q uaternary scientists need to be able to situate a large num ber and wide variety o f observations
and fin d s fro m the North Sea in th e ir proper geological setting in ord er to translate these fin d s into
inform a tion about past landscapes and th e ir inhabitants (e.g. Gaffney et al., 2009). The North Sea
area is a critical region to answer some m ajor open questions in the fields o f palaeoanthropology
and Pleistocene studies as NW Europe was at the edge o f hom inin expansion durin g its evolutionary
stages th ro ug h the Pleistocene (e.g. Parfitt e t a!., 2005; Roebroeks, 2005; Parfitt et a!., 2010). As
the archaeological richness o f the Rhine-Thames part o f the North Sea is becoming more and more
evident, studies providing proper regional geological co nte xt fo r all stages o f the Palaeolithic are
needed. The increasingly larger volum es o f Pleistocene sedim ents th a t are mined from the North
Sea, in particular fo r sand extra ction , makes this even more im p orta nt. A spectacular exam ple is the
rare fin d o f a Neanderthal skull fra g m e n t fro m o ff the coast o f the Netherlands (Hublin e ta !., 2009).
C om paratively little was known about the geological setting o f the fossil. Local stratigraphical
co nte xt is m issing, and the Pleistocene landscape co nte xt is o nly known in broad and general
term s.
The Hijma e t al. (2012) paper provides a regional fram ew ork to place sites from older and younger
Palaeolithic periods in a continuous landscape e volutionary fram ew ork. The paper integrated
geological data fro m the Belgian, Dutch and British onshore and offshore, and visualizes th is fo r
critical periods as palaeogeographical scenario maps (Figures 1,2). Two o f these maps contrast the
situation in the interglacials o f the Middle Pleistocene, before and after the A nglian/E lsterian
glaciation. Before th is glaciation (in the Pleistocene up to 500,000 years ago) a wide land bridge
existed between England and Belgium even during marine highstands. The land bridge was much
narrower, but not yet fu lly removed and not yet lowered to below sea level on its northern flank, in
the interglacials o f the period a fte r 500,000 years ago, up to the Saalian glaciation (150,000 years
ago) at the end o f the M iddle Pleistocene. Erosive action by m elt w ater from th is ice age finished the
jo b o f rem oving the land bridge, and replaced it w ith an axial valley th a t connected rivers from the
North Sea to those o f the English Channel. A th ird map shows the Strait o f Dover, Southern Bight
and more n orthe rly parts o f the North Sea firs t fu lly connected durin g the sea level high stand o f
the last interglacial (Eemian, 120,000 years ago). A fo u rth maps shows the axial valley re-emerged
w ith sea level high stand o f the Last Glacial, w ith the rivers Rhine, Thames, Meuse and Scheldt all
jo in e d and routed south as the so-called Channel River. For archaeology, the differences in river
network, coastal co nfig uratio n and erosion base owing to the stage wise removal o f high stand
situations have strong archaeological im plications via (i) changes m igration routes o f herds o f
herbivores on which hom inins preyed, (ii) changes in availab ility o f flin t raw m aterials in England,
northern France and Belgium, and (iii) progressively deepening valleys and increasing loess
sedim entation changing the ways in which archeological sites fo rm and preserve (site taphonom y).
Geologically, it provides 10 cross-sections th a t allow to trace several generations o f Late Pleistocene
valley systems o f the Rhine tow ards the British-Belgian sector and the Strait o f Dover: connecting
the datable d epositional record o f the Q uaternary North Sea basin into the geom orphological
traceable record in T ertia ry and Mesozoic substrate o f the fo rm e r landbridge.
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= - 1 ,000,000 - 500,000 yr ago
= Bavelian, Cromerian Complex
= MIS 21, 19, 17, 15, 13 highstands
E A R L Y M ID D L E P L E IS T O C E N E
( p a la e o g e o g r a p h ic s c e n a rio )
F igure 1. A n in te rg la c ia l h ig h s ta n d s itu a tio n d u rin g
th e e a rly M id d le P le istocene as re p e a te d ly o c c u rrin g
b e tw e e n 1 and 0 .5 Ma (i.e. ‘ Bavelian, C ro m e ria n
C o m p le x '; MIS 21, 19, 17, 15, 13) sh o w in g
highstand: shallow sea
lowstand: delta of
Rhine-Meuse-Thames
re p e a te d ly g la c ia te d he a dlan d s.
SOUTHERN NORTH SEA
Holocene drow ning o f the North Sea
The largest river in the southern North Sea is
the Rhine-Meuse system. The paper by Hijma
V.u ’ h f- r n North
coastal plain
and Cohen (2011) describes the tran sition
from Late Pleistocene valley to Holocene delta
plain o f th is system, onshore and offshore o f
the present coast. The critical period in this
tra n sitio n was the tim e between 9000 and
6000 years ago. A series o f palaeogeographical maps is presented, based on
w e a ld
m apping and dating at very high data
densities and com pletely covering the system
- reaching a level o f detail unsurpassed in the
H /.r.'T i
w orld. The map series shows a Rhine-MeusePalaeogeography at times of high sea level
Scheldt estuary offshore o f Rotterdam at
Uplands, valleys, coastsand shallowseas
Resistant strata
Rivers and structural _geomorphology
.. .
I
I Shalow sea and coast
I
I
ancient terraces I
1 Palaeocene marine clay / " * ■ Evident from deposits, geomorphology
9000 years, which rapidly drowns and shifts
■ ■ Active floodplainsand deltas I
I Higher uplands
I-1-!! Cretaceous chalk
. Inferred river courses
Relative young terraces
0 Early palaeolithic sites
Palaeozoic outcrop
* \ v # Cuestas, major faults, basin hinge line
landwards, loses the Scheldt as a feeding
river, starts to lose the Rhine too, develops a m uddy and organic backfill and em bryonic barrier
systems, e xpo rts sedim ent to the coastal barrier system and tid al inlets to the north o f the
drow ning estuary. A ll o f th is happens over a tim e period o f 2500 years, leading up to stabilisation
o f barrier coastal system around 6000 years ago. This resolves the transgressive stage o f coastal
evolution o f the North Sea to sim ilar level o f detail and process explanation as the later Holocene
stages. To have closed th is fo rm e r knowledge gap is especially im p o rta n t because it allows studying
landscape evolution o f the North Sea’s river valleys from the last ice age to its river m ouths and
coasts o f the Holocene in a continuum .
-".M,
.
- S y fW n lE r c
The position o f the Rhine-Meuse-Scheldt system on the wide, low -gradient continental shelf th a t the
North Sea basin provides is o f particular im portance. It has made th a t base-level changes is a
d om in a nt control on sedim entation fo r relative short periods o f tim e w ith in glacial cycles only, and
kicks in relative late in the interglacial. Plenty o f accom m odation space is left to fill by te rre stria l
deposition by rivers, and the system can be seen to go th ro ug h changes in sedim entary style in
response to hinterland clim atic changes. This holds fo r fu ll glacial co nd itio ns o f the Last Glacial
M axim um and tim es before; w ith beginning w arm ing in the Late-glacial, and w ith continued
w arm ing
o f the Early Holocene, and the later North Sea flo o r and coastal plain base overly
considerable preserved valley surface area from each o f these tim e periods.
IL A
P IL F
OU
f'Fb N
^ TI F
AT
I F
b K
b I'^
lb TI U
N Fb -. IL A
Ao
L il.L A
A PUIA
A IL
( p a la e o g e o g ra p h ic s c e n a rio )
= '80-000
* 20,000 yrEar1y
ago and
= last
glacialpieniglacial
until LGM
= Weichse|ian:
Midd|e
= m is 4 -3 -2 lowstand
L ATE P L E IS T O C E N E : F IR S T 5 0,000 Y E A R S
= Eem’ )
( p a la eogeogra phic scenario )
= MIS 5e highstand; MIS 5d-a falling stage
= Last Interglacial and begin Last Glacial
SOUTHERN NORTH SEA
Saalian Drenthe
SOUTHERN NORTHSEA
E a s t A n g lia
E a s t A n g lia
/
'
B ank'S ,,
delta plain
B ro w n
J
B a n k " -.,!
since -6 0 ka
Rhine-Meuse
Thames ____
W e a ld
W eald
:dover
STRAIT
A rto is
L-A rtois
Channel
river
A rd e n n e s
A rd e n n e s |
Eemian highstand
□
Shallowsea andcoast
■ Rrvervalleys4 deltas
PM Uplands (higher, lower)
Last Glacial Rhine-Thames-Channel river and surroundings
Resistant strata
Rivers and structural geomorphology
[T~] Last-active braidplams
|-----1Uplands
p q Palaeocene marine day/— Evident fromdeposits, geomorphology
q
pieniglacial terraces, with site r j Brief ice sheet presence E 3 Cretaceous chalk
/ - Possible additional temporarypaths
Emerged seafloor and
□ Brief blocked drainage m Palaeozoc outcrop * \*. Cuestas, may* faulls. basin hinge line
formervalleys(higher, lower)
Early Weichselianfalling stage
Resistant strata
Rivers and structural geomorphology
□
Shallowsea andcoast
B Palaeocene marine d a y /- Evident fromdeposits, geomorphology
H J Activeriver deltas
E 3 Cretaceouschalk
Inferred continuations I delta dnrenuons
I 1 submerged/emergingvalley
Palaeozoic outcrop \V»» cueslas' major faulls. basin hingeline
F igu re 2. Le ft: T he h ig h s ta n d s itu a tio n o f th e Last In te rg la c ia l (MIS 5e) and th e fa llin g sta g e (sub-) h ig h s ta n d s o f
th e Early W e ich se lia n (MIS 5d-a). T h e m ap is re p re s e n ta tiv e f o r th e la te M id d le P a la e o lith ic (1 3 0 -8 0 ka ago). R ight:
T he lo w s ta n d s itu a tio n fo r th e c o ld e s t p a rt o f th e Late P leistocene, th e W e ich se lia n Early and M id d le P ie n ig la cia l,
up to th e Last G lacial M a x im u m (_ 8 0 -2 0 ka ago).
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For the transgression, it is shown th a t the base o f the coastal prism records three stages, (i) From
the m illennium before 8.45 ka BP, deltaic flu via l environm ents o f extensive wetlands bury Lateglacial and Early Holocene valley parts, (ii) The m illennium a fte r 8.45 ka begins w ith a rapid
drow ning event. Thereafter, marked erosion in the coastal zone, an em bryonic barrier system th a t
stepped back many kilom eters, increasing tid al am plitudes in the estuaries, and bay-head delta
developm ent in upper estuarine environm ents are seen, (iii) The period between 7.5 and 6.3 ka BP
saw the Rhine step backward and d ivert away from th is fo rm e r m outh th ro u g h m ultiple avulsion,
and eventually saw the coastal barrier system stabilize its position, m arking the high stand to have
commenced.
Sea level ju m p in g 8,450 years ago
The paper by Hijma and Cohen (2010) describes the evidence fo r the pulse o f accelerated sea-level
rise th a t occurred at 8.45 ka. Both the tim in g and the m agnitude o f th is sea-level ju m p were
q uantified d irectly fro m precise sea-level data harvested from the Rhine delta. Evidence fro m
te rrestrial, glacial, and global clim ate m odel reconstructions suggests th a t th is sea-level ju m p was
caused by huge am ounts o f m eltw ater release in the fin al stages o f existence o f proglacial lake
Agassiz in North-Am erica. Drainage o f th is lake in fro n t o f the m elting Laurentide ice sheet is
nowadays w idely regarded to have caused the 8.2 ka cooling event, th a t was particularly significant
in the N orthern Hemisphere (see T örn qvist en Hijma, 201 2 fo r a recent overview). The chronology o f
the m eltw ater pulse, however, so fa r had been based on marine data o f lim ited dating accuracy,
which had placed it at ca. 8470 ± 300 yr. The data fro m Rotterdam, observed at considerable
distance fro m the release site, specify a bru p t sea-level rise to have comm enced 8450 ± 44 yr ago
and shows sea level markers fro m before and a fte r the ju m p to be separated 4 meters vertically.
Between 8500 to 8300 years ago, sea level is shown to have ju m p e d a 2.11 ± 0.89 m, in a dd itio n to
background relative sea-level rise over th a t tw o century period (1.95 ± 0.74 m). In o the r words: in
the North Sea basin, in these special centuries sea level rise was double th a t o f w hat was seen in the
centuries before and after. Considerable areas o f land transform ed to shallow sea at th is tim e. Full
marine connection between the Southern Bight and German Bight was established, and the wave
and tid al regime as we know it to da y spun up. The tem poral acceleration o f the sea-level ju m p
catalyzed these tran sfo rm atio n s from subaerial to drowned conditions. The event set back coast
lines and river m ouths, and is recorded as the transgressive surface at the base o f the Rhine-Meuse
coastal prism . Due to the event-nature o f the sea level ju m p , the dia chro ne ity o f th is transgressive
surface is stron gly suppressed in the critical area o f the later coastal zone. The fo u r meters o f very
rapid rise were follow ed by some 10 meters o f gradually decelerating fu rth e r rise in the m illen n ia which drowned the basal transgressive contact relatively deep, preserving it relative widely.
Corrected fo r g ravitational effects w ith distance to release site, the m agnitude at Rotterdam
translates to a global-averaged eustatic sea-level ju m p th a t is double the size o f previous estim ates
(3.0 ± 1.2 m versus 0 .4 -1 .4 m). The discrepancy suggests e ither a coeval A n ta rctic c o n trib u tio n or,
more likely, a previous underestim ate o f the to ta l Am erican lake drainage. Besides the global
clim atological and sea level signal links, the event is o f circum -oceanic stratigraphical relevance
deltaic and coastal successions w orldw ide, event to the e xten t th a t ju m p ’s transgressive contact is
o f use in fo rm a lly d ivid in g the Early Holocene from the Middle Holocene (Hijm a en Cohen, 2010;
Cohen en Hijma, 201 3). Reference to the sea-level acceleration results since 2010 includes the IPCC
fifth assessment report, prospection and discovery o f Early M esolithic sites below Rotterdam
harbour extension (Weerts et al., 2012), and renewed atte ntio n to the tim e-depth interval in Asian
deltas, notably th a t o f the Yangtze (Wang et al., 201 3).
C ontinuous geological coverage
The series o f papers address critical tim e-periods th a t to g e th e r describe the com ing into existence
o f the North Sea as we know it today. This provide a considerably updated fram ew ork fo r the
m acro-evolution o f the North Sea during the last m illion years. In our appreciation, o ld er macroevolutionary overviews in the past often suffered from gaps. Late Pleistocene chapters w ould cover
the glacial m axim um pre do m in an tly and Holocene chapters the last 6000 years. Early Holocene
te rre stria l co nditions were not considered to be relevant fo r sedim entation. Understanding o f the
Holocene transgression o f the North Sea was underdeveloped and projection rather than based on
mapping. The papers avoid such gaps. Clouds o f sea-level rise data were collected from before,
from durin g and from after the sea-level ju m p in the 2010 paper. The fu ll tra n sitio n from glacial
river valley, th ro u g h adaptive phases o f clim atic am elioration o f the glacial-interglacial tran sition ,
th ro ug h transgressive estuarine situations, to the eventual barrier coast and delta plain is described
as a continuous sto ry in the 2011 paper, h ig h lig h tin g aspects o f geological-geom orphological
inheritance in the developm ent besides external changes. The progressive nature o f the erosive
tran sfo rm atio n o f the Belgian-English landscape to the Southern Bight open sea is h ighlighted in the
201 2 paper.
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Such e ffo rts enable next steps o f research, such as studying rates o f coastal processes under
considerable sea-level rise (e.g. fo r coastal geom orphologists), regional geological correlation and
com parison along the North Sea (e.g. fo r stratigraphers), and the m erging o f te rre stria l and offshore
geological fram ew orks (e.g. fo r p aleolithic archeologists). Furtherm ore, the publications have
renewed international atte ntio n fo r the Southern North Sea region as a geological reference area fo r
shelf, delta and sea-level research. Lastly, the papers contain new insights th a t are provoking and
renewing research cooperation between onshore and offshore groups from the countries around the
North Sea.
Research group acknowledgm ents
The papers fro m 2010 were part o f the Ph.D.-research o f the main applicant between 2005-2009 at
Utrecht U niversity (The Netherlands). The w ork was done in close cooperation w ith Dr. Kim Cohen
(Utrecht University; Deltares and TNO Geological Survey o f The Netherlands). The 2012 paper is the
outcom e o f a post-doctoral stay at Leiden U niversity w ith professor Wil Roebroeks, w ith Dr. Wim
W esterhoff and Dr. Freek Busschers (TNO Geological Survey o f The Netherlands) as fu rth e r
collaborators.
References
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