Response to Comment on “Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply”

Abstract

Tolstoy reports the existence of a characteristic 100 thousand year (ky) period in the bathymetry of fast-spreading seafloor but does not argue that sea level change is a first-order control on seafloor morphology worldwide. Upon evaluating the overlap between tectonic and Milankovitch periodicities across spreading rates, we reemphasize that fast-spreading ridges are the best potential recorders of a sea level signature in seafloor bathymetry. We acknowledge the clear distinction made by Tolstoy (1) between the ideas put forward in (2) and the hypothesis central to (3). In (3), it is proposed that seafloor relief is caused primarily by changes in the height of volcanic constructions on Milankovich time scales, which reflect a control of sea level cycles on mid-ocean ridge (MOR) magma supply. Meanwhile, (1, 2) point out that seafloor eruption rates may be extremely sensitive to small external perturbations such as orbital stresses. In particular, they suggest that-in addition to sea level cycles modulating overall MOR magma supplyeccentricity cycles may modulate rates of volcanic extrusion on the seafloor. A 100-thousand-year (ky) characteristic periodicity in the bathymetry of the fast-spreading Southern East Pacific Rise (SEPR) is presented to support this idea. In (2), however, Tolstoy does not imply that such processes are the primary cause for the abyssal hill fabric of global seafloor, which was the focus of our initial study (4).A characteristic periodicity of 100 ky at the SEPR, with a full spreading rate of 14.7 cm/year, translates into a seafloor length scale of 7.4 km, which is considerably larger than the characteristic 1-to 3-km spacing of normal faults measured at fast-spreading ridges (5, 6). The biggest tectonic scarps (~50 m) in such settings correspond to axis facing faults that accommodate the small fraction of plate separation not taken up by crustal emplacement (7,8). Additional, more closely spaced faults form in response to flexural stresses as the lithosphere migrates away from the axial high, but these generally produce smaller scarps (≤20 m) (5, 9-11). Overall, the spectral signature of the tectonic fabric of fast-spreading seafloor should consist of several distributed peaks at periodicities near and below~40 ky (Fig. 1). Such peaks are present in the bathymetric spectrum of the SEPR but are much less energetic than the 100-ky peak (2). This 100-ky peak could therefore be the signature of a periodic (but nontectonic) process capable of creating coherent relief on top of the fault-induced fabric.Based on these observations of fault geometry and distribution, our models suggest that topography-building processes at MORs act as low-pass filters-they only record fluctuations in melt supply on time scales longer than the cutoff period set by intrinsic properties of the lithosphere and magmatic accretion zone. Tolstoy rightfully points out that this cutoff period generally decreases with spreading rate, making fastspreading ridges the best candidates to efficiently record a climatic modulati...

Causes of Oceanic Crustal Thickness Oscillations Along a 74‐M Mid‐Atlantic Ridge Flow Line

“…There are no significant peaks in the bathymetry power spectrum at any of the Milanković‐cycle periods (100, 41, and 23 kyr) as reported by Crowley et al (), supporting the idea that ocean floor bathymetry formed at slow‐spreading MORs is not sensitive to fluctuations in melt supply triggered by sea level change (Goff, ; Goff et al, ; Olive et al, , ).…”

No Evidence for Milankovitch Cycle Influence on Abyssal Hills at Intermediate, Fast, and Superfast Spreading Rates

“…This hypothesis is consistent with previous studies (Bohnenstiehl & Carbotte, ; Malinverno & Cowie, ) that successfully model fault scaling parameters (spacing, length, and throw) as random distributions governed by scaling laws. The work of Olive et al () also provides supporting evidence; after stacking conjugate corridor profile of the data used in the Crowley et al () study, they found that that the spectral peaks that had been attributed to Milankovitch cycles disappeared. Hence, even with just two independent profiles, it is clear that the main abyssal hill peaks and valleys do not stack coherently as a function of age.…”

“…The Milankovitch cycle hypothesis has garnered significant attention (Conrad, 2015;Hand, 2015), in large part because the potential implications are far-reaching, that is, that abyssal hills, the most common landform on Earth (Menard, 1967), could provide a record of global sea level fluctuations and climate history over many tens of millions of years. However, the hypothesis has been the focus of substantial criticism (Goff, 2015;Macdonald, 2015;Olive et al, 2015) and resulted in vigorous discussion (Crowley et al, 2015b;Huybers et al, 2016;Olive et al, 2016aOlive et al, , 2016bTolstoy, 2016). There are four important criticisms of the Milankovitch cycle hypothesis and the basis for its assertion: (1) Abyssal hills are dominantly fault-bounded structures that form kilometers off axis, rather than volcanic constructs, as evidenced by decades of observational studies (Macdonald, 2015;Macdonald et al, 1996); therefore, the primary "signal" of abyssal hill topography is a record of MOR faulting rather than magmatic output; (2) lava emplacement on fast-spreading MORs tends to occur as sheet flows often extending several kilometers from the zero-age axis (e.g., Escartín et al, 2007;Macdonald et al, 1989;Mitchell, 1995;Soule et al, 2005), which complicates the relationship between crustal age and distance from the axis over temporal scales appropriate for the Milankovitch cycle hypothesis (Goldstein et al, 1994;Macdonald, 2015); (3) globally, there is an overall decrease in abyssal hill widths with spreading rate (Goff et al, 1997), which is consistent with a faulted origin but contrary to the Milankovitch cycle hypothesis prediction of an increase with spreading rate (Goff, 2015;Olive et al, 2015); and (4) the observational evidence of the Crowley et al (2015a) study in particular relied on a single profile and thus should be considered as anecdotal evidence for a hypothesis that makes predictions on regional or even global scales.…”