5.1. SST Variability
 Although the small anomalies reconstructed in both cores suggest Carolina Slope SST has not changed markedly in the past 1500 years, the subtle changes show some significant differences from hemispheric temperature. During the MCA-LIA transition, where our age model is robust, hemispheric temperatures begin cooling toward the LIA after ∼1000 A.D. (Figure 7d). LIA cooling also begins at this time in the Gulf of Mexico [Richey et al., 2007] and near the Great Bahama Bank [Lund and Curry, 2006] (Figure 7e), suggesting similar responses to common forcings.
Figure 7. (a) One hundred year binned 59GGC SST and one sigma standard error. (b) Cosmic ray induced ionization (CRII, gray) [Usoskin et al., 2008] and total solar irradiance (black) [Bard et al., 2000]. (c) NAO z scores [Trouet et al., 2009]. (d) Northern Hemisphere (NH) surface temperature [Mann et al., 2008]. (e) One hundred year binned Gulf of Mexico (GOM) SST and one sigma standard error (black) [Richey et al., 2007] and 100 year binned Great Bahama Bank (GBB) SST and one sigma standard error (gray) [Lund and Curry, 2006]. Thick lines in Figures 7b–7d are 100 year bins. The approximate timing of the MCA and LIA are marked, as is the switch to a cooling trend near 1000 A.D. in NH temperature.
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 In contrast, Carolina Slope SSTs exhibit a pronounced warm period near 1250 A.D., well after other regional records have started to cool (Figure 7a). It is unclear if this warm interval reflects the continuation of higher MCA SSTs or a warm reversal following an initial cooling near 1000 A.D. In any case, our 59GGC reconstruction demonstrates that the Carolina Slope experienced SST variability that is not evident in hemispheric [Mann et al. 2008] and other regional [Lund and Curry, 2006; Richey et al., 2007] proxy records.
 Carolina Slope warmth during and after the MCA may reflect the regional influence of centennial-scale NAO-like circulation, possibly in response to solar forcing. Proxy reconstructions of solar irradiance describe variability during the past two millennia as an 11 year cycle superimposed upon a slowly varying baseline [Lean et al., 1995]. Although considerable uncertainty exists in how to scale proxy data to radiative forcing, most reconstructions show patterns of centennial-scale baseline variability [Bard et al., 2000; Bard and Frank, 2006; Usoskin et al., 2008] that are similar to Carolina Slope SST (Figure 7b). Periods of weak solar forcing generally correspond to cooler SSTs, while periods of enhanced solar forcing, including a period near 1250 A.D., correspond to warmer SSTs.
 Attempts to isolate the influence of irradiance on climate indicate that the solar variability shown in Figure 7b would cause Carolina Slope SST anomalies of only a few tenths of a degree [Lean and Rind, 2008]. However, irradiance-induced changes to atmospheric circulation may significantly amplify the climatic impact of solar anomalies. Global climate models suggest irradiance anomalies are capable of causing atmospheric circulation patterns resembling modern NAO phases [Shindell et al., 2001]. Positive irradiance anomalies warm low-latitude SSTs creating stratospheric temperature gradients that ultimately give rise to enhanced tropospheric westerlies and a circulation pattern similar to the positive phase of the NAO [Shindell et al., 2001]. A circulation resembling the negative phase of the NAO occurs during periods of reduced irradiance. Although the modern NAO is an interannual phenomenon that is most pronounced in winter, its circulation pattern has been used to describe centennial to millennia-scale climate variability during the Holocene [e.g., Sachs, 2007]. To avoid confusion with the modern NAO, we refer to low-frequency circulation patterns resembling the NAO as NAO-like.
 We suggest higher solar irradiance from ∼700–1250 A.D. may have shifted atmospheric circulation toward a more positive NAO-like state, while negative NAO-like conditions prevailed after 1250 A.D. This possibility is supported by high-resolution reconstructions that indicate a large-scale reorganization of atmospheric circulation near 1250 A.D. resembled the negative NAO (Figure 7c) [Laird et al., 1996; Trouet et al., 2009]. If modern, decadal-scale NAO relationships are applicable to our low-frequency record, a positive NAO-like circulation prior to ∼1250 A.D. may have forced positive SST anomalies near the Carolina Slope [Visbeck et al., 1998]. These anomalies may have been caused by a northward shift of the zero wind stress curl line over the Atlantic, which reduced the ocean to atmosphere heat flux along the Carolina Slope and warmed SSTs [Battisti et al., 1995; Marshall et al., 2001]. If this effect was sufficiently large near the Carolina Slope, it may have temporarily offset the cooling observed throughout the Northern Hemisphere and in other portions of the low-latitude western North Atlantic.
 Carolina Slope SST cooled as NAO-like circulation became less positive after ∼1250 A.D., eventually reaching its coldest values near 1500 A.D. when the NAO was entering its negative phase. Although 59GGC does not robustly resolve the timing or the trend of SST variability since the LIA, the rise in mean SST near the core top hints that our data may be consistent with increased solar irradiance, a more positive NAO-like circulation, and anthropogenic warming in recent centuries.
 Solar irradiance-induced changes in NAO-like circulation provide a plausible explanation for our reconstructed SST variability, but similarities between the AMO and Carolina Slope SST since the 1850s (Figures 2a and 2b) suggest AMOC variability may also be important. Changes in the AMOC are characterized by SST anomalies of a single sign throughout the North Atlantic [Knight et al., 2005; Stouffer et al., 2006]. Although the amplitude of these anomalies is damped along the Carolina Slope, a weaker AMOC is characterized by cooler Atlantic SSTs North of the equator while the opposite holds for a stronger AMOC.
 Despite considerable uncertainty in reconstructions of the AMOC during the past two millennia, many records suggest overturning was stronger during the MCA and weaker during the LIA. Carbon isotopes from a Bermuda Rise sediment core suggest northern-sourced deepwater was enhanced from ∼0–1000 A.D., but was significantly reduced during the LIA [Keigwin and Boyle, 2000]. Similarly, evidence suggesting that vigorous flow of Iceland-Scotland Overflow Water near 1000 A.D. became weaker during the LIA [Bianchi and McCave, 1999] is broadly consistent with a ∼10% reduction in northward transport through the Florida Straits between 1000 A.D. and 1500 A.D. [Lund et al., 2006]. Although not conclusive, these proxy records clearly demonstrate the possibility that changes in the AMOC could have contributed to climate variability during the past 2000 years [Denton and Broecker, 2008].
 Estimates of subpolar SST from planktic foraminifera show a pronounced warming around 1250 A.D. [Andersson et al., 2003] that is nearly synchronous with our Carolina Slope record and consistent with the pattern expected from an enhanced AMOC (Figure 8b). North Atlantic drift ice records (Figure 8c) also suggest AMOC variability, possibly forced by solar irradiance, was a prominent control on late Holocene Atlantic climate [Bond et al., 2001]. However, other high-latitude North Atlantic proxy records show little evidence for warming near 1250 A.D. [Cronin et al., 2010]. Assuming such differences cannot be attributed to age model uncertainties, they do not support a synchronous, basin-wide SST anomaly forced by AMOC variability. Rather, we suggest that any influence of the AMOC on Carolina Slope SST variability during the past two millennia was likely part of complex ocean-atmosphere coupling. For example, coupled model simulations indicate sustained positive NAO-like circulation patterns can enhance the AMOC [Delworth and Dixon, 2000], and can cause an in-phase warming between the subtropical and subpolar Atlantic on multidecadal timescales [Visbeck et al., 1998] that is also generally consistent with Figure 8.
Figure 8. (a) One hundred year binned 59GGC SST and one sigma standard error. (b) One hundred year binned planktic foraminifera–based subpolar North Atlantic SST and one sigma standard error [Andersson et al., 2003]. (c) Subpolar drift ice based on a multicore stack of percent hematite stained grains [Bond et al., 2001].
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5.2. δ18Osw Variability
 Carolina Slope δ18Osw suggests variable but generally fresh MCA conditions that became more saline during the LIA (Figure 9a). This pattern differs from Gulf of Mexico [Richey et al., 2007] and Great Bahama Bank δ18Osw reconstructions, but is similar to a record from the Dry Tortugas [Lund and Curry, 2006] (Figure 9b). Because models suggest the amount and the δ18O of precipitation near our site remained relatively unchanged during the late Holocene [LeGrande and Schmidt, 2009], we suggest changes in Carolina Slope δ18Osw reflect the tropical Atlantic evaporation/precipitation balance. Cariaco Basin percent titanium [Haug et al., 2001], a proxy for Atlantic Intertropical Convergence Zone (ITCZ) precipitation, suggests a Northerly ITCZ freshened the tropical Atlantic during the MCA leading to a negative salinity anomaly. Advection of this low-salinity anomaly in surface currents may then explain relatively low Carolina Slope δ18Osw during the MCA.
 If changes in Carolina Slope δ18Osw are caused by advected anomalies, the volume of water transported to the site may also be important. Evidence that Florida Current transport was reduced by ∼10% during the LIA [Lund et al., 2006] (Figure 9d), hints that reduced transport may have contributed to more positive Carolina Slope δ18Osw anomalies at that time. However, the Carolina Slope remained relatively fresh throughout the decline in transport from 800 to 1400 A.D., suggesting that volume of transport is of secondary importance to δ18Osw variability.
5.3. Salinity Effects on Mg/Ca
 Recent evidence suggests that the high salinity at our subtropical site could influence G. ruber Mg/Ca [Ferguson et al., 2008; Mathien-Blard and Bassinot, 2009; Arbuszewski et al., 2010]. To assess the importance of this effect, we applied a recent calibration that simultaneously solves for mean annual SST and salinity using Mg/Ca, δ18Oc and ΔCO32−, where ΔCO32− is the difference between the carbonate ion concentration and saturation at a given depth in μmol kg−1 [Arbuszewski et al., 2010]. We estimated 59GGC ΔCO32− to be 24.62 μmol kg−1 based on the 1250 m horizon of Anderson and Archer's  Atlantic carbonate ion concentration profile. This value was considered to be constant. Because the calibration of Arbuszewski et al.  solves for SST and salinity, the later was converted to δ18Osw using equation (3) to more easily compare the two methods.
 Alternate SST, salinity and δ18Osw estimates average 25.82 ± 0.66°C, 36.28 ± 0.11 psu and 1.19 ± 0.04‰, respectively. The comparable values using equations (1)–(3) are 25.14 ± 0.40°C, 36.21 ± 0.55 psu and 1.17 ± 0.18‰, respectively (Figure 10). The trends of the alternate SST and δ18Osw estimates are generally similar to those based on traditional methods, and they do not fundamentally change our conclusions. Although SSTs calculated from Arbuszewski et al.  suggest the MCA was significantly warmer than 59GGC's core top, the onset of LIA cooling still lags with respect to other hemispheric and regional temperature records. Similarly, alternate δ18Osw estimates exhibit the same pattern of variability, and still support a possible link with the tropical ITCZ.
Figure 10. (a) Average 59GGC SST based on the calibration of Arbuszewski et al.  at each depth after applying our age model (thin line) and in overlapping bins (thick line) with associated one sigma standard error (dashed line). The binned mean from Figure 6 is shown for comparison (gray line). (b) As in Figure 10a for 59GGC δ18Osw converted from salinity using equation (3).
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 A notable difference between the methods is the amplitude of SST and δ18Osw variability. Using Arbuszewski et al.  increases the standard deviation (1σ) of mean SST by 0.16°C and decreases that of δ18Osw variability by 0.14‰. Given that SST has varied by more than 1°C in recent decades (Figure 2), both methods estimate a reasonable range of SST values. In contrast, the amplitude of δ18Osw variability reconstructed using equations (1) and (2) (Figure 6) may be unrealistically large given that the estimated Carolina Slope salinity variability of ∼0.2 psu (Figure 2) is equivalent to only ∼0.06‰ based on equation (3). Although we cannot rule out large salinity variations in the past, the damped salinity and δ18Osw estimates calculated from Arbuszewski et al.  seem more realistic when compared to modern variability.