Eutrophication of Chesapeake Bay Kemp, W. M.; Boynton, W. R.; Adolf, J. E. ...
Marine ecology. Progress series,
11/2005, Letnik:
303
Journal Article
Recenzirano
Odprti dostop
This review provides an integrated synthesis with timelines and evaluations of ecological responses to eutrophication in Chesapeake Bay, the largest estuary in the USA. Analyses of dated sediment ...cores reveal initial evidence of organic enrichment in ~200 yr old strata, while signs of increased phytoplankton and decreased water clarity first appeared ~100 yr ago. Severe, recurring deep-water hypoxia and loss of diverse submersed vascular plants were first evident in the 1950s and 1960s, respectively. The degradation of these benthic habitats has contributed to declines in benthic macroinfauna in deep mesohaline regions of the Bay and blue crabs in shallow polyhaline areas. In contrast, copepods, which are heavily consumed in pelagic food chains, are relatively unaffected by nutrient-induced changes in phytoplankton. Intense mortality associated with fisheries and disease have caused a dramatic decline in eastern oyster stocks and associated Bay water filtration, which may have exacerbated eutrophication effects on phytoplankton and water clarity. Extensive tidal marshes, which have served as effective nutrient buffers along the Bay margins, are now being lost with rising sea level. Although the Bay’s overall fisheries production has probably not been affected by eutrophication, decreases in the relative contribution of demersal fish and in the efficiency with which primary production is transferred to harvest suggest fundamental shifts in trophic and habitat structures. Bay ecosystem responses to changes in nutrient loading are complicated by non-linear feedback mechanisms, including particle trapping and binding by benthic plants that increase water clarity, and by oxygen effects on benthic nutrient recycling efficiency. Observations in Bay tributaries undergoing recent reductions in nutrient input indicate relatively rapid recovery of some ecosystem functions but lags in the response of others.
The relative paucity of recent post‐subduction environments globally has meant that, so far, little is known about tectonic processes that occur during and after subduction termination, as previously ...convergent tectonic plates adjust to the new stress regime. The region of Southeast Asia that now encompasses northern Borneo has been host to two sequential episodes of subduction—both now terminated—since the mid‐Paleogene. It is expected that these processes will have left signatures in the fabric of the upper mantle, which are manifest in the form of seismic anisotropy. We investigate the evidence for, and alignment of, anisotropic fabrics by measuring the splitting of a family of teleseismic shear phases. These observations provide a measure of the orientation of the effective anisotropic elastic tensor, in the form of the orientation of the fast shear‐wave polarization, ϕ, and add constraints on the strength of the anisotropic fabric, in the form of the delay time, δt. We observe two principal trends across northern Borneo that appear to be confined to the lithosphere. These patterns are likely related to tectonic processes associated with subduction, continental collision, and oceanic basin formation, events that can exert primary influence on the formation of post‐subduction settings.
Plain Language Summary
This study is concerned with understanding what happens to the upper 200 km of the Earth when subduction—the process by which one plate pushes beneath another and sinks into the Earth's interior—stops. We measure a property of the rock in the upper 200 km called seismic anisotropy, which tells us how fast earthquake waves move when traveling or polarized in one direction compared to another. Seismic anisotropy can inform us about both large‐scale plate tectonic events in recent geological history (10s of millions of years) and present‐day deformation. Northern Borneo has undergone two phases of active subduction followed by termination in the last 25 million years, making it one of the few places on Earth where we can explore this important stage of the subduction cycle. We find that tectonic compression and extension events related to termination and post‐subduction processes have left strong imprints in the upper 100 km of the Earth, leaving little‐to‐no remnants of signals we might have expected to observe from the active phase of subduction.
Key Points
New catalog of shear‐wave splitting measurements from a dense network in post‐subduction setting
Two trends in the fast orientations, corresponding to fabric generated by subduction termination and post‐subduction processes
Seismic anisotropy is limited to the lithosphere beneath northern Borneo, with no strong signal in radial anisotropy of simple asthenospheric flow
Subduction polarity reversal (SPR) is a key subduction initiation mechanism often associated with arc‐continent collision zones. Northern Borneo has long been recognized as a location where ...sequential but opposing subduction zones were present in the Miocene, but has not been examined in the context of SPR. Here, we exploit teleseismic data from northern Borneo to investigate crustal thickness variations using Virtual Deep Seismic Sounding (VDSS). Our results reveal a thick crustal root beneath the Crocker Range and an area of relatively thin crust in the southeast, which appears to extend northeast into the Sulu Sea, where back‐arc rifting behind the younger subduction zone developed. Overall, our findings are consistent with predictions from numerical models of SPR involving arc‐continent collision, but with several important differences—including a substantial mountain range and more limited back‐arc rifting that can be attributed to northern Borneo being an example of SPR involving continent‐continent collision.
Plain Language Summary
Northern Borneo lies in the heart of Southeast Asia and was assembled by a complex series of tectonic events over the last 40 million years. Principle among these was the subduction of the Proto South China Sea plate beneath its northwest continental margin, which ended in continent‐continent collision. Subduction of the Celebes Sea plate in the southwest followed, which terminated ∼9 million years ago. In this study, we exploit seismic data collected in northern Borneo to construct a new crustal model of the region, that robustly constrains the crust‐mantle boundary. We find that crustal thickness variations are consistent with a subduction polarity reversal (SPR) event induced by continent‐continent collision, with thicker crust underlying the collision zone and thinner crust related to extension driven by retreat of the younger subduction zone. Our results have important implications for the study of SPR and subduction initiation and the processes that influence post‐subduction tectonic settings.
Key Points
Crustal thickness map of northern Borneo obtained from Virtual Deep Seismic Sounding
Evidence of crustal thinning indicates Sulu Sea extension propagated into northern Borneo during the late Miocene
Variations in crustal thickness consistent with subduction polarity reversal induced by continent‐continent collision
SUMMARY
Information on fault zone structure is essential for our understanding of earthquake mechanics, continental deformation and seismic hazard. We use the scattered seismic wavefield to study the ...subsurface structure of the North Anatolian Fault Zone (NAFZ) in the region of the 1999 İzmit and Düzce ruptures using data from an 18-month dense deployment of seismometers with a nominal station spacing of 7 km. Using the forward- and back-scattered energy that follows the direct P-wave arrival from teleseismic earthquakes, we apply a scattered wave inversion approach and are able to resolve changes in lithospheric structure on a scale of 10 km or less in an area of about 130 km by 100 km across the NAFZ. We find several crustal interfaces that are laterally incoherent beneath the surface strands of the NAFZ and evidence for contrasting crustal structures either side of the NAFZ, consistent with the presence of juxtaposed crustal blocks and ancient suture zones. Although the two strands of the NAFZ in the study region strike roughly east–west, we detect strong variations in structure both north–south, across boundaries of the major blocks, and east–west, parallel to the strike of the NAFZ. The surface expression of the two strands of the NAFZ is coincident with changes on main interfaces and interface terminations throughout the crust and into the upper mantle in the tomographic sections. We show that a dense passive network of seismometers is able to capture information from the scattered seismic wavefield and, using a tomographic approach, to resolve the fine scale structure of crust and lithospheric mantle even in geologically complex regions. Our results show that major shear zones exist beneath the NAFZ throughout the crust and into the lithospheric mantle, suggesting a strong coupling of strain at these depths.
The mantle transition zone (MTZ) structure beneath the Ethiopian Rift and Afar is mapped using receiver functions. The 410 discontinuity is flat and regionally depressed by 30–40 km, most likely due ...to a hot (≥+250 °C) and slow (average δVS > 3 %) upper mantle. The 660 discontinuity is shown to have variations in depth (665–705 km) over short length scales (<200 km). This results in a MTZ with a ‘normal’ average thickness of 244 km, (i.e., within error of the observed global average). However, local thickness variations (<230 km to >260 km) indicate possible compositional/chemical heterogeneities and elevated ambient temperatures near the base of the MTZ. These observations provide evidence for a link between the low velocity anomalies of the Ethiopian upper mantle and the African Superplume in the lower mantle.
Key Points
Ethiopian mantle transition zone structure is mapped with receiver functions
Mantle transition zone has a mean of 244 km but is locally <230 and >260 km
Thickness changes are attributed to adjacent pyrolite and piclogite composition
SUMMARY
The deep crustal structure beneath the North Sea is poorly understood since it is constrained by only a few seismic reflection and refraction profiles. However, it is widely acknowledged that ...the mid to lower crust plays important roles in rift initiation and evolution, particularly when large-scale sutures and/or terrane boundaries are present, since these inherited features can focus strain or act as inhibitors to extensional deformation. Ancient tectonic features are known to exist beneath the iconic failed rift system of the North Sea, making it an ideal location to investigate the complex interplay between pre-existing regional heterogeneity and rifting. To this end, we produce a 3-D shear wave velocity model from transdimensional ambient seismic noise tomography to constrain crustal properties to ∼30 km depth beneath the North Sea and its surrounding landmasses. Major North Sea sedimentary basins appear as low shear wave velocity zones that are a good match to published sediment thickness maps. We constrain relatively thin crust (13–18 km) beneath the Central Graben depocentres that contrasts with crust elsewhere at least 25–30 km thick. Significant variations in crustal structure and rift symmetry are identified along the failed rift system that appears to be related to the locations of Laurentia–Avalonia–Baltica palaeoplate boundaries. We constrain first-order differences in structure between palaeoplates; with strong lateral gradients in crustal velocity related to Laurentia–Avalonia–Baltica plate juxtaposition and reduced lower crustal velocities in the vicinity of the Thor suture, possibly representing the remnants of a Caledonian accretionary complex. Our results provide fresh insight into the pivotal roles that ancient terranes can play in the formation and failure of continental rifts and may help explain the characteristics of other similar continental rifts globally.
SUMMARY
Debate is ongoing as to which tectonic model is most consistent with the known geology of southeast Australia, formerly part of the eastern margin of Gondwana. In particular, numerous ...tectonic models have been proposed to explain the enigmatic geological relationship between Tasmania and the mainland, which is separated by Bass Strait. One of the primary reasons for the lack of certainty is the limited exposure of basement rocks, which are masked by the sea and thick Mesozoic–Cenozoic sedimentary and volcanic cover sequences. We use ambient noise tomography recorded across Bass Strait to generate a new shear wave velocity model in order to investigate crustal structure. Fundamental mode Rayleigh wave phase velocity dispersion data extracted from long-term cross-correlation of ambient noise data are inverted using a transdimensional, hierarchical, Bayesian inversion scheme to produce phase velocity maps in the period range 2–30 s. Subsequent inversion for depth-dependent shear wave velocity structure across a dense grid of points allows a composite 3-D shear wave velocity model to be produced. Benefits of the transdimensional scheme include a data-driven parametrization that allows the number and distribution of velocity unknowns to vary, and the data noise to also be treated as an unknown in the inversion. The new shear wave velocity model clearly reveals the primary sedimentary basins in Bass Strait as slow shear velocity zones which extend down to 14 km in depth. These failed rift basins, which formed during the early stages of Australia–Antarctica break-up, appear to be overlying thinned crust, where high velocities of 3.8–4.0 km s−1 occur at depths greater than 20 km. Along the northern margin of Bass Strait, our new model is consistent with major tectonic boundaries mapped at the surface. In particular, we identify an east dipping velocity transition zone in the vicinity of the Moyston Fault, a major tectonic boundary between the Lachlan and Delamerian orogens, which are part of the Phanerozoic accretionary terrane that makes up eastern Australia. A pronounced lineament of high shear wave velocities (∼3.7–3.8 km s−1) in the lower crust of our new model may represent the signature of relict intrusive magmatism from failed rifting in the early stages of Australia–Antarctica break-up along a crustal scale discontinuity in the Selwyn Block microcontinent which joins Tasmania and Victoria.
Earthquakes at Campi Flegrei have been low magnitude and sparse since 1985, denying onshore monitoring observations of their usual source for structural constraint: seismic tomography. Here we used ...continuous seismic records from 2011–2013 to reconstruct period‐dependent Rayleigh wave group velocity maps of the volcano. The Neapolitan Yellow Tuff rim faults bound high‐velocity intracrater products of historical eruptions, which act as a barrier for deep fluid migration. The anomaly with lowest group velocity is aseismic and corresponds to the portion of a fluid storage zone that was fractured during the 1984 volcanic unrest under Pozzuoli town. Earthquake locations show that fluids migrate from this area toward the Solfatara and Pisciarelli fumaroles along shallower low‐velocity fractures. The aseismic anomaly is likely fed by a deep‐seated offshore magmatic source. Its spatial relation with regional dynamics and observations from historical unrests mark the area as the most likely feeder pathway for fluid and magmatic inputs from depth.
Plain Language Summary
We used the seismic noise produced by the seashore at Campi Flegrei volcano to detect magma and hot fluids under its surface. The area where likely magmatic fluids are entering the volcano was part of a reservoir, broken during the 1980s and intersected by strong earthquakes that forced the population to move outside of Pozzuoli town. This area is a feeder pathway as it connects deeper sources of magma, likely active since the 1980s, to the upper part of the volcano and feeds hot fluids to the hydrothermal systems. The injections of either magma or fluids that fractured the reservoir in the 1980s have changed Campi Flegrei characteristics, but the feeder pathway is still the most probable pathway for deeper magma and fluids to enter the volcano subsurface. Hazardous fluids migrate from the feeder pathway to fumaroles at Solfatara and Pisciarelli, west of the Napoli city center, following fractures and faults mapped at surface.
Key Points
Rayleigh wave tomography of Campi Flegrei caldera used continuous ambient noise data recorded during the 2011–2013 volcanic unrest
Low velocities map the aseismic pathway opened during the 1984 volcanic unrest, feeding heat and fluids to the shallow hydrothermal systems
High‐velocity intracrater domes and structural faults control the feeder extension and fluid migration to fumaroles
We use teleseismic recordings from a dense array of seismometers straddling both strands of the North Anatolian Fault Zone to determine crustal thickness, P/S velocity ratio and sedimentary layer ...thickness. To do this, we implement a new grid search inversion scheme based on the use of transfer functions, removing the need for deconvolution for source normalization and therefore eliminating common problems associated with crustal‐scale receiver function analysis. We achieve a good fit to the data except at several stations located in Quaternary sedimentary basins, where our two‐layer crustal model is likely to be inaccurate. We find two zones of thick sedimentary material: one north of the northern fault branch, and one straddling the southern branch. The crustal thickness increases sharply north of the northern strand of the North Anatolian Fault Zone (NAFZ), where the fault nearly coincides with the trace of the Intra‐Pontide Suture; the velocity ratio changes across the southern fault strand, indicating a change in basement composition. We interpret these changes to indicate that both strands of the NAFZ follow preexisting geological boundaries rather than being ideally aligned with the stress field. The thick crust north of the northern NAFZ strand is associated with low topography and so is inconsistent with simple models of isostatic equilibrium, requiring a contribution from mantle density variations, such as possible loading from underthrust Black Sea oceanic lithosphere.
Key Points
New seismic method reveals crustal variations across North Anatolian Fault
Fault strands follow existing geological terrane boundaries
Complex isostatic state due to heterogeneous crust and mantle density
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