57 Vol. 10(2), 2001
O verview
The A rctic M id-O cean Ridge Ex- pedition (A M O RE 2001) returned in early O ctober 2001 after an incredibly successful nine-w eek study of G akkel Ridge and its surrounding basins in the high A rctic. A M O RE 2001 w as an international effort involving tw o ice- breakers: PFS Polarstern, from the A lfred W egener Institute in Brem er- haven, G erm any, and the new U .S.
icebreaker, U SCG C H ealy. It w as H ea- ly’s m aiden scientific voyage, and she proved to be an excellent icebreaker and scientific platform . This historic and highly successful expedition far exceeded anyone’s expectations and w ent w ell beyond the goals set forth by InterRidge in charting and sam - pling G akkel Ridge. Som e of the high- lights of the expedition are:
- B asalts and peridotites w ere recovered from over 200 sites w ithin and near the axis of G akkel Ridge, about three tim es as m any sites as w ere planned.
- H ydrotherm al plum es w ere discovered and sam pled along this ultraslow spreading ridge.
- A high-resolution, w ell-navigated m ap of the ridge w as unexpectedly produced using the hull-m ounted m ultibeam sonar system s, w hich w orked far better in the ice than
anticipated.
- Successful seism ic m easurem ents show ed that crustal thickness varies strongly along the axis of G akkel R idge, m ost likely according to distinct volcanic centers.
- The crustal thickness in the N ansen Basin does not follow theoretical m odels, w hich predict thin crust at slow spreading rates. The crust thickens tow ards the G akkel Ridge.
Introduction
G akkel Ridge is an end-m em ber of the global spectrum of m id-ocean ridg- es in m any respects, and offers a unique com bination of characteristics (e.g.
spreading rate, geographical location, obliquity, segm entation) w hich m ay control the com position of the erupt- ed m agm as, the crustal thickness and the presence of hydrotherm al activi- ty. Its spreading rate is by far the slow est of any m id-ocean ridge and varies by a factor of tw o along its length. A M O RE 2001 has thus greatly extended the range of values over w hich w e can investigate the relation- ships betw een ridge properties and spreading rate. G akkel Ridge has an exceptionally deep rift valley, and the thinnest know n crust for a norm al ridge (<4 km ). It has no large offsets,
so it allow s exam ination of the roles of ridge obliquity (transform faults) ver- sus m antle upw elling in causing ridge segm entation. G akkel Ridge is far from the Indian O cean, and therefore al- low s separation of the effects of spreading rate from the anom alous Indian O cean m antle source in the geochem istry of basalts. A nalysis of a few sm all basalt and peridotite sam - ples from G akkel Ridge suggests the extents of m elting m ay be very low (M üheet al., 1997; H ellebrand et al., in press). This has im plications for the ratio of peridotite to basaltic crust that m ay be present in the ridge axis.
W hile so far there is little doubt on the existence of thin crust in the rift valley, the situation off-axis is differ- ent. Past observations and a recent study (W eigelt & Jokat, 2001) indicate that there m ight be no sim ple relation betw een spreading velocity and crus- tal thickness aw ay from the G akkel rift valley. A lthough spreading velocity decreases, sparse seism ic refraction data and gravity m odeling suggest a thickening of the oceanic crust. It is not clear w hether this observation is typical or if it represents only local variations in the com position of the oceanic crust. In any case it challeng- es currently accepted theoretical m od- els. M aybe G akkel Ridge represents a
R esults of the A rctic M id-O cean R idge Expedition - A M O R E 2001 – Seafloor Spreading at the Top of the W orld
A M O RE Shipboard Scientific Parties of U SCG C H ealy and RV Polarstern (Peter M ichael
1, Jörn Thiede
2, Charles Langm uir
3, W ilfried Jokat
2, H enry D ick
4, Jon Snow
5, D avid G raham
6, E. W eigelt
2,
Steven G oldstein
3, Richard M ühe
7, H enrietta Edm onds
8, O . Ritzm ann
2, G regory K urras
9, A nnette Buechl
5, Linda K uhnz
10, Stefan G auger
2, K erstin Lehnert
3, M . Schm idt-A ursch
2, Jeffrey Standish
4, T.
Schm idt
2, Jam es Broda
4, B. Schram m
11, J.H atzky
2, G ad Soffer
3) (partial list)
1 D ept. of G eosciences, The U niversity of Tulsa, 600 S. College Ave., Tulsa O K 74104, U SA
2 Alfred W egener Institute for Polar and M arine Research, Colum busstrasse, 27568 Brem erhaven, G erm any
3 Lam ont-D oherty Earth O bservatory, Palisades, NY 10964-8000, U SA
4W oods H ole O ceanographic Institution, W oods H ole, M A 02543, U SA
5M ax-Planck Institut für Chem ie, Abt. G eochem ie, Postfach 3060, D -55020 M ainz, G erm any
6College of O ceanographic and Atm ospheric Sciences, O regon State U niversity, Corvallis, O R 97331, U SA
7Institut für G eowissenschaften, U niversity of Kiel, G eologie, G erm any
8 U niversity of Texas at Austin, M arine Science Institute, Port Aransas, TX, 78373, U SA
9 D ept. of M arine G eology & G eophysics, U niversity of H awaii, H onolulu,U SA
11U niviversiät Brem en – G eowissenschaften, Postfach 330 440, 28334 Brem en, G erm any
International Ridge-Crest Research: A rctic R idges
InterRidge News 58
threshold spreading environm ent, w here existing global m odels fail in general.
H ow the m antle beneath the A rc- tic O cean is related to the m antle be- neath the northernm ost A tlantic O cean and the rest of the planet, and how it m ay have been influenced by the nearby continents are additional basic questions that w ill be addressed by geochem ical study of the igneous rocks. G akkel Ridge is our sole oppor- tunity to sam ple this portion of the earth’s interior.
G akkel R idge
G akkel Ridge stretches 1800 km across the Eurasian Basin of the A rc- tic O cean, all of it beneath A rctic sea ice (Fig. 1). It is the m ost rem ote and slow est spreading portion of the glo- bal m id-ocean ridge system . To the w est it passes via Lena Trough and the M olloy Fracture Zone into K nipov- ich Ridge, the m ost northern part of the M A R. Its eastern end runs into the continental m argin of the Laptev Sea, w here rifting continues (D rachev et al., 1998). Spreading rates decrease from 1.33 cm /yr (full rate) at the w est- ern end to 0.63 cm /yr at the eastern end in the Laptev Sea. Spreading is
nearly orthogonal to the strike of the ridge and there is only one m ajor off- set in the ridge axis at about 60°E (K ovacs et. al., 1985).
C ruise O peration
The ships left Trom sø July 31 and approached G akkel Ridge from east of Svalbard at about 15°E (Fig. 1). The ships first joined G akkel Ridge at 20°E after the seism ic reflection survey crossing the entire N ansen Basin. Both ships then traveled w estw ard along the axis to 8°W perform ing bathym et- ric m apping and sam pling and acquir- ing seism ic refraction data along axis betw een the sam pling stations. The ships then sam pled the rift axis and w alls intensively as they returned eastw ard to 20°E, operating som ew hat independently because of favorable ice conditions. The northern and southern w alls of the rift valley w ere m apped during this return. D uring all seism ic reflection experim ents in the N ansen and A m undsen basins as w ell as the seism ic refraction profiles along the G akkel Ridge, both ships operated jointly. H ere, H ealy led the convoy to break ice for Polarstern that tow ed the stream er and the airguns (Fig. 1). For both transects in the N ansen and
A m undsen basins this setup w as crit- ical for the excellent data quality achieved. Because of ice conditions, the latter transect took place at 72°E instead of the prim ary geographical objective w hich w as to have been a long transect perpendicular to the ridge at 85°E. A t the end of the survey, both ships visited the N orth Pole, w here a brief celebration w as held. U SCG C H ealy returned to G akkel Ridge at 87°E for intensive sam pling of a recent lava flow (Edw ards et al., 2001) w hile Po- larstern returned to G akkel Ridge along the seism ic survey’s path to the w est and occupied heat flow stations in the basin. The ships rejoined on G akkel Ridge at 72°E for the return trip w est- w ard along the ridge that involved intensive sam pling and m ore bathy- m etric m apping, w ith a w ide angle seism ic study carried out concurrent- ly. Ice and fog conditions w orsened around Septem ber 11, so sam pling becam e m ore difficult and som e tar- gets w ere forsaken. Still, H ealy and Polarstern sam pled and m apped som e- w hat independently but in a coordi- nated program until the tim e at w hich they left the ice around 24°E on Sep- tem ber 27, 2001. U SCG C H ealy re- turned to Trom sø on O ctober 2, 2001
0
10 20 30 40 50
60 70 80 90 100 110
120 130 140 170 150
170 180 150160 130 140 110 120 90100 70 80 60 40
30
20
10
72 72
73
73
74
74
75
75 76
76
76
76
77
77 77
77
78 78
78
78
79 80 80
81 81
82 82
83 83
84 84
85 8585
86
86 87
0
10 20 40 50
60 70 90 100 110
120 130 140 170 150
170 180 150 130 140 110 120 90100 70 80 60 40
30
20
72 72
73
73
74
74
75
75 76
76
76
76
77
77 77
77 78
78
79 80 80
81 81
82 82
83 83
84 84
85 8585
86 86
nautical miles
0 kilometers 1000
Gakkel Ridge Greenland
Russia Amundsen Basin
Nansen Basin
Svalbard
Laptev Sea Lomonosov Ridge
Figure 1. M ap of the seafloor of the A rctic O cean show ing the cruise tracks of U SCG C H ealy and PFS Polarstern during the A M O RE 2001 expedition.
International Ridge-Crest Research: A rctic R idges: M ichael,et al., cont...
59 Vol. 10(2), 2001
w hile Polarstern w ent to Brem erhaven on O ctober 7.
N ew bathym etric m ap of G akkel R idge produced
Surprisingly, the ships’ bottom m apping sonar system s (Seabeam 2112 on H ealy and H ydrosw eep on Po- larstern) w ere able to generate superb m aps of the seafloor even w hile the ships w ere breaking ice. The bathy- m etric results far exceeded our expec- tations. The total surveyed region covers ~1000 km of the axis from 8°W (Lena Trough) to 88°E, providing the first data for the w estern G akkel Ridge.
The resolution of these data is signif- icantly better than previously existing bathym etry from SC IC EX (Cochran et al., in prep.) and reveals geologic detail critical to understanding the segm entation and volcanic and tec- tonic processes of this ultra-slow spreading M O R. The new bathym etry data show three distinct m agm atic- tectonic regions w ithin the area m apped.
R ock recoveries
There w as som e doubt about w hether w e w ould be able to dredge in ice covered w aters. A fter a steep learn- ing curve, the success rate for dredg- ing w as fairly high. Flexibility in choos- ing targets w as im portant, and in a few cases, large ice floes kept us aw ay from entire regions. Each dredge operation had to be carefully set up and planned, using leads through the ice pack and taking into account ice drift velocities. In addition to dredges on both ships, U SCG C H ealy em - ployed w ax cores to recover glass and PFS Polarstern had a TV -G rab. These m ethods required less open w ater to succeed. Rock sam ples w ere recov- ered from m ore than 200 sites along the axis and flanks of G akkel Ridge, m ostly by dredging.
M ore than 120 basalt glass sam - ples w ere analyzed on board U SCG C H ealy for m ajor elem ents, Sr and Ba by direct current plasm a spectrom etry.
Because the cruise track encom passed a double-pass along m ost of the ridge, the on board data perm itted testing of hypotheses form ulated on the first
pass by further sam pling on the sec- ond pass. M odels for the effect of decreasing spreading rate on m elt com position that predicted progres- sively sm aller extents of m elting at greater depths eastw ard along the ridge w ill be tested using these data.
Forty-six thin sections and hun- dreds of hand sam ples of m antle peri- dotites w ere exam ined during the course of the expedition. M ost of these peridotites are altered 60-90% , like m ost abyssal peridotites. Som e how ever are stunningly fresh, con- taining no detectable serpentine in thin section. The distribution of m an- tle rock types is sim ilar to that from other m id-ocean ridges, but peridot- ites from G akkel R idge seem to have undergone low degrees of partial m elting in accordance w ith theoreti- cal predictions.
H ydrotherm al activity along G akkel R idge
M iniature A utonom ous Plum e Recorders from Ed Baker of N O A A PM EL w ere used on dredges and rock cores to identify sites of hydrotherm al venting through light scattering and tem perature anom alies associated w ith hydrotherm al plum es. In all, there w ere 118 M A PR deploym ents from H ealy and 19 from Polarstern. Several plum es w ere found, and several had corre- sponding tem perature anom alies. O n board analysis and interpretation of the M A PR data w ere used to target CTD /rosette deploym ents, w hich w ere collected from H ealy at six stations along the G akkel Ridge. Plum e w ater sam ples w ere collected for M n, m eth- ane, and 3H e to confirm the hydrother- m al nature of the light scattering anom - alies and provide som e estim ate of source strength. U nw eathered hydro- therm al sulfide chim neys w ere dredged at one site. In addition, a potential fossil hydrotherm al upflow zone as evidenced by abundant epidosite rocks w as also dredged from a tecton- ically uplifted portion of the ridge flank.
Biological specim ens
M any of the 98 recovered dredges by U SCG C H ealy contained biologi-
cal sam ples from the benthos and w ater colum n. A nim als, m ollusk shells, fossils, associated rocks, and all other evidence of biological activity w ere collected. O rganism s w ere preserved using m ultiple m ethods for planned m orphological and genetic studies. A surprising num ber of dredges yielded sponges and shrim p. Though the sam - pling w as not biologically targeted, the recovered anim als are uniquely valuable to science. Sessile species hold clues to the m inim um age of re- cent lava flow s and sulfide deposits.
If the organism s are hydrotherm ally associated, their distributions w ill in- dicate or confirm active venting areas along the ridge, and could extend bi- ogeographic inferences into another ocean basin. Pending funding, com - plete taxonom ic sorting of sam ples and species identifications w ill be conducted, new species w ill be fully described, and correlations betw een biological distributions and extant venting w ill be investigated.
G eophysical Experim ents
To provide a consistent geo- physical/petrological m odel for the super-slow G akkel R idge, sufficient inform ation on the crustal thickness and the com position of the upper m antle beneath the rift valley and its flanks is required. Several different geophysical m ethods w ere applied to m eet these objectives. B oth con- ventional ship-based experim ents like seism ic reflection experim ents as w ell as m easurem ents located on drifting ice floes w ere conducted. The results are briefly review ed here.
Seism ic Reflection Experim ents. To determ ine the crustal structure of the Eurasian Basin north and south of G akkel R idge, tw o long seism ic transects in A m undsen and N ansen basins w ere acquired. A 24 l airgun cluster in com bination w ith a 300 m long stream er (48 channels, 6.25 m group spacing) w as used. In addition, 36 sonobuoys w ere deployed in order to provide inform ation on sedim ent and crustal velocities for a depth con- version of the seism ic data. A ll three profiles provided excellent data and m ost of the oceanic basem ent w as
International Ridge-Crest Research: A rctic R idges: M ichael,et al., cont...
InterRidge News 60
clearly im aged after processing. The sonobuoys provided signals from even deeper levels of the oceanic crust and in a few cases, signals from the M oho are visible. This allow ed a m in- im um estim ate of the crustal thick- ness. G ravity m odeling of the transects w ill provide m ore reliable crustal m od- els than in the past.
Seism ic Refraction Experim ents. To investigate the crustal thickness along the rift valley of G akkel Ridge both ships had to w ork together. For this type of reconnaissance survey, only a few stations w ere deployed along each profile. In case of reverse shoot- ing at m axim um tw o seism ic data ac- quisition units w ere deployed on ice floes to record the airgun signals.
D uring profiling, U SG CC H ealy led the convoy, w hile RV Polarstern tow ed an airgun array (in total 24-l) to gener- ate the acoustic signals. Crustal thick- ness w as m easured at 18 different locations. A ll stations w orked w ith- out problem s. M ost of the record sec- tions show clear Pn arrivals from the crust/m antle boundaries w ith veloci- ties betw een 7.8 and 7.9 km /s. The crustal thickness along the rift valley varies betw een 2 and 6 km .
G ravity m easurem ents. A fixed m ounted gravity m eter K SS31 onboard the FS PO LA RSTERN gathered grav- ity data during the entire cruise. The instrum ent w orked w ithout any prob- lem s during the entire cruise. H arbor values w ere taken in Trom sø and Brem erhaven.
H elicopter based M agnetics. This program intended to fly a detailed m agnetic survey across the rift valley of G akkel Ridge. U nfortunately m ost of the planned survey could not be conducted, due to constantly foggy w eather conditions. M easurem ents w ere perform ed during only 14 days of the cruise. M agnetic data w ere gath- ered for a total flight tim e of 56 hours (4480 nm ) w ith a line spacing of 2 km across the ridge. The data are of good quality and w ere flow n across prom i- nent bathym etric features, so a contri- bution tow ards better understanding of spreading processes along the G akkel Ridge can be expected.
H eat Flow m easurem ents. Thirty
eight heat flow m easurem ents w ere m ade at fourteen heat flow stations along the rift valley of G akkel Ridge, and seven along an off-axis seism ic transect into the A m undsen Basin.
H ere, good control for the sedim ent thickness w as provided by the seis- m ic reflection data acquired on the w ay to Lom onosov Ridge. In the rift valley, it w as difficult to find sedim ent patches of a sufficient extent to per- form the m easurem ents. The Para- sound data clearly show ed that sm all volcanoes covered m ost of the sea- floor w ith only a few sedim ents in betw een.
Rem ote M agnetotelluric Experi- m ents and Seism ological A rray. The deploym ent of the seism ological and m agnetotelluric stations on the ice faced tw o problem s. The constantly bad flight conditions in the beginning of the cruise in com bination w ith the relatively fast sam pling of the petrol- ogy program did not allow the sta- tions to be deployed a reasonable distance to the ship. The risk involved in finding the stations after several days of deploym ent and w ith flight distances of m ore than 50 N M w as too large. Secondly, the tim e of 3 hours plus lim ited flight w indow s needed to construct one M T station restricted the num ber of instrum ents.
Five M T-experim ents w ere con- ducted along G akkel Ridge to investi- gate the conductivity of the earth’s crust and the m antle below this m id- ocean ridge. The stations w ere recov- ered after 3 - 9 days. Critical to the interpretation of these data is the rota- tion of the ice floe on w hich the instru- m ents are located. A lthough the floes show ed significant drift paths, their rotation w as not so strong. So the instrum ents acquired reasonable data for m ost of the deploym ent periods.
W hile the crustal thickness along G akkel Ridge w as determ ined by seis- m ic refraction experim ents, seism o- logical data are necessary to probe the upper m antle. For this experim ent a m obile netw ork consisting of 3-4 sta- tions w as deployed on an ice floe. The deploym ent of the array w as m ostly finished in three hours. The RefTek recording units had alm ost no failures
during their deploym ent on the floes.
A first view of the seism ological data show ed that teleseism ic as w ell as local events w ere recorded. The m ost spectacular quakes w ere recorded from the Pacific-A ntarctic ridge w ith sufficient S/N ratio. A careful data analysis w ill show to w hich extend local seism icity along the ridge w as recorded.
R eferences
Coakley, B. J., and J. R. Cochran. G ravity Evidence of very thin crust at the G akkel Ridge (A rctic O cean). Earth Planet. Sci. Lett., 162,81-95, 1998.
D rachev, S. The Laptev Sea rifted continental m argin: M odern know ledge and unsolved questions.
Int’l.Conf.Arctic M argins (ICAM III) A bstracts, Celle, G erm any (abstract).
48-49,1998.
Edw ards, M .H ., G .J. K urras, M . Tolstoy, D .R. Bohnenstiehl, B.J. Coakley, and J.R. Cochran. Evidence of recent volcanic activity on the ultraslow - spreading G akkel Ridge.Nature409, 808-811,2001.
H ellebrand, E., R. M ühe, and J.E. Snow . M antle m elting beneath Gakkel Ridge (A rctic O cean): A byssal peridotite spinel com positions. (in preparation, Chem ical G eology)
Jakobsson,M .,Cherkis,N.,W oodward,J., Coakley, B.J. and M acnab, R., A new grid of Arctic bathym etry: A significant resource for scientists and m apm akers.
EO S Trans. Am . G eophys. U nion, vol.
81,pp.89-96,2000.
K ovacks, L.C., C. Bernero, G .L. Johnson, R.H.Pilger,S.P.Srivastava,P.T.Taylor, G .E. V ink, and P.R. V ogt. Residual m agnetic anom aly chart of the A rctic O cean region. G eological Society of America,M ap and Chart Series,M C53, scale 1:6,000,000.1985.
M ühe, R.K ., H . Bohrm ann, D . G arbe- Schönberg and H.Kassens.E-M ORB glasses from the G akkel Ridge (A rctic Ocean) at 87°N: evidence for the Earth’s m ost northerly volcanic activity.Earth Planet. Sci. Lett.152, 1-9, 1997.
W eigelt,E.and Jokat,W .(2001).Pecularities of roughness and crustal thickness of oceanic crust in the Eurasian Basin, A rctic O cean, G eophys. J. Int., 145, 505-516.
